<PAGE>
 
                                                                    EXHIBIT 10.8

                 THE NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY
                           720 East Wisconsin Avenue
                          Milwaukee, Wisconsin  53202


                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and The Northwestern Mutual Life Insurance Company.  Terms
used herein with initial capital letters that are not otherwise defined shall
have the meanings ascribed to such terms in the Standby Commitment Letter.

     The parties hereto hereby agree as follows:

     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization of the Debtors in the form attached as Exhibit A to
     the Merger Agreement,
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     with such amendments and modifications thereto as are
     made in a manner consistent with clause (e) of Section 5 hereto (such plan
     of reorganization being referred to herein as the "Plan"), (i) cash and
     (ii) shares of its Common Stock, par value $.01 per share ("Existing Arch
     Common Stock"); (c) holders of unsecured non-priority claims against the
     Debtors ("Unsecured Claims"), to the extent such Unsecured Claims are
     Allowed (as defined in the Plan), will receive pursuant to the Plan (i) an
     aggregate of 14,344,969 shares of Existing Arch Common Stock and (ii)
     rights to purchase ("Rights") an aggregate of 108,500,000 shares of
     Existing Arch Common Stock ("Rights Shares") for $2.00 per share in cash
     (the "Subscription Price"); (d) holders of claims arising under or relating
     to the Credit Agreement, dated December 4, 1995, as amended, among
     MobileMedia and the other parties thereto ("Secured Claims"), to the extent
     such Secured Claims are Allowed, will receive pursuant to the Plan cash in
     an amount equal to 100% of such claims; (e) all of the outstanding equity
     interests in MobileMedia and Parent will be canceled without consideration
     and Parent will be dissolved; and (f) the commitments under the DIP Loan
     Agreement will terminate and all amounts owed under or in respect of the
     DIP Loan Agreement will be paid in full in cash.  Arch will conduct the
     Stockholder Rights Offering, in which it will issue to holders of Buyer
     Stock Stockholder Rights to acquire an aggregate of 44,893,166 shares of
     Existing Arch Common Stock, and, immediately following the Combination,
     Arch will issue warrants entitling the holders thereof to purchase shares
     of Existing Arch Common Stock ("Arch Participation Warrants"), with such
     Arch Participation Warrants to be issued pursuant to, and to have the terms
     set forth in, a warrant agreement in the form attached as Exhibit B-1 to
     the Merger Agreement (the "Arch Participation Warrant Agreement"), to the
     stockholders of Arch to the extent any Stockholder Rights issued to such
     Stockholder Rights Holder were not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be distributed, in each case as permitted by Section 3 below,
     and (ii) the Rights
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     sold or transferred by the Standby Purchaser or the Rights distributed in
     respect of Unsecured Claims held by it as of the date hereof that are
     hereafter sold or transferred by the Standby Purchaser are not exercised
     prior to the expiration thereof (at which time such Rights will be void and
     will no longer be exercisable), to purchase for cash (based upon the
     Subscription Price payable upon exercise of such Rights) the Rights Shares
     underlying such unexercised Rights, to the extent that the aggregate
     purchase price therefor, together with the aggregate Subscription Price
     payable upon exercise of Rights exercised as contemplated by clause (a)
     above, does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto; and
                               -------        

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights"). Any Rights that remain unexercised
     upon expiration thereof will be deemed to
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     be "Section 3(b) Rights" up to, but not exceeding, the amount of Untracked
     Rights. The Section 3(b) Rights shall be exercised as follows prior to the
     application of Section 1(c) above and Section 1(c) of the Other Standby
     Purchase Commitments: (A) the Standby Purchaser and the Other Standby
     Purchasers will first be given the opportunity to purchase for cash (based
     on the Subscription Price payable upon exercise of such Rights) the Rights
     Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------                      
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     designees) certificates representing the Arch Participation Warrants
     contemplated by Section 7 below.

     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.

     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     shares of Existing Arch Common Stock issuable upon conversion of any such
     shares of the Arch Class B Common Stock or exercise of any such Arch
     Participation Warrants (the securities referred to in the foregoing clauses
     (i), (ii) and (iii) are referred to herein as the "Registrable
     Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;

     13.  Section 6.  The proviso at the end of Section 6 of the Standby
          ---------                                                     
Commitment Letter is hereby amended in its entirety to read as follows:

     provided, however, that the Standby Purchaser may not assert the condition
     --------  -------                                                         
     contained in this clause (iii) if each of the Unaffiliated Standby
     Purchasers (other than Credit Suisse First Boston Corporation), acting in
     good faith, shall have waived the condition in clause (iii) of Section 6 of
     the Other Standby Purchase Commitment to which such Unaffiliated Standby
     Purchaser is a party.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h) and 10(i) at the end thereof, with such
Sections 10(g), 10(h) and 10(i) to read in their entirety as follows:

     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may take any action in its capacity as a member of the Official
     Committee of Unsecured Creditors appointed in connection with the Chapter
     11 proceeding (the "Committee") that the Standby Purchaser determines, in
     good faith, after consultation with counsel (which may be counsel to the
     Committee), is required by its fiduciary duties as a member of the
     Committee under applicable law.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) The Standby Purchaser agrees that the waiver by Credit Suisse First
     Boston Corporation of the conditions contained in clause (ii) of Section
     5(a) and clause (ii) of Section 5(k) of the Other Standby Purchase
     Commitment to which it is a party, pursuant to Section 10(i) of such Other
     Standby Purchase Commitment, shall be deemed a good faith waiver for
     purposes of Sections 5(a)(ii) and 5(k)(ii) above.

     17.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".

     18.  New Section 13(c).  Article 13 of the Standby Commitment Letter is
          -----------------                                                 
hereby amended to add the following Section 13(c) of the end thereof:

     (c) Each of the parties hereto acknowledges and agrees that,
     notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may not waive, or otherwise abrogate its right to assert, the
     conditions contained in clause (ii) of Section 5(a), clause (ii) of Section
     5(k) and clause (iii) of Section 6 other than in connection with a
     scheduled Closing.

     19.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 16 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:

     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     20.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        

     21.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     22.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     23.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     24.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.


                              Very truly yours,

                              THE NORTHWESTERN MUTUAL LIFE
                                 INSURANCE COMPANY

                              By: /s/ 
                                 -------------------------------
                              Name:
                              Its:

                              Address:   720 E. Wisconsin Avenue
                                         Milwaukee, WI 53202
                                         Attention: Securities Department
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   -------------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer

With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   -------------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>
<PAGE>
 
                 THE NORTHWESTERN MUTUAL LIFE INSURANCE COMPANY
                     for its Group Annuity Separate Account
                           720 East Wisconsin Avenue
                          Milwaukee, Wisconsin  53202


                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and The Northwestern Mutual Life Insurance Company for its
Group Annuity Separate Account. Terms used herein with initial capital letters
that are not otherwise defined shall have the meanings ascribed to such terms in
the Standby Commitment Letter.

     The parties hereto hereby agree as follows:

     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     of the Debtors in the form attached as Exhibit A to the Merger Agreement,
     with such amendments and modifications thereto as are made in a manner
     consistent with clause (e) of Section 5 hereto (such plan of reorganization
     being referred to herein as the "Plan"), (i) cash and (ii) shares of its
     Common Stock, par value $.01 per share ("Existing Arch Common Stock"); (c)
     holders of unsecured non-priority claims against the Debtors ("Unsecured
     Claims"), to the extent such Unsecured Claims are Allowed (as defined in
     the Plan), will receive pursuant to the Plan (i) an aggregate of 14,344,969
     shares of Existing Arch Common Stock and (ii) rights to purchase ("Rights")
     an aggregate of 108,500,000 shares of Existing Arch Common Stock ("Rights
     Shares") for $2.00 per share in cash (the "Subscription Price"); (d)
     holders of claims arising under or relating to the Credit Agreement, dated
     December 4, 1995, as amended, among MobileMedia and the other parties
     thereto ("Secured Claims"), to the extent such Secured Claims are Allowed,
     will receive pursuant to the Plan cash in an amount equal to 100% of such
     claims; (e) all of the outstanding equity interests in MobileMedia and
     Parent will be canceled without consideration and Parent will be dissolved;
     and (f) the commitments under the DIP Loan Agreement will terminate and all
     amounts owed under or in respect of the DIP Loan Agreement will be paid in
     full in cash. Arch will conduct the Stockholder Rights Offering, in which
     it will issue to holders of Buyer Stock Stockholder Rights to acquire an
     aggregate of 44,893,166 shares of Existing Arch Common Stock, and,
     immediately following the Combination, Arch will issue warrants entitling
     the holders thereof to purchase shares of Existing Arch Common Stock ("Arch
     Participation Warrants"), with such Arch Participation Warrants to be
     issued pursuant to, and to have the terms set forth in, a warrant agreement
     in the form attached as Exhibit B-1 to the Merger Agreement (the "Arch
     Participation Warrant Agreement"), to the stockholders of Arch to the
     extent any Stockholder Rights issued to such Stockholder Rights Holder were
     not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     distributed, in each case as permitted by Section 3 below, and (ii) the
     Rights sold or transferred by the Standby Purchaser or the Rights
     distributed in respect of Unsecured Claims held by it as of the date hereof
     that are hereafter sold or transferred by the Standby Purchaser are not
     exercised prior to the expiration thereof (at which time such Rights will
     be void and will no longer be exercisable), to purchase for cash (based
     upon the Subscription Price payable upon exercise of such Rights) the
     Rights Shares underlying such unexercised Rights, to the extent that the
     aggregate purchase price therefor, together with the aggregate Subscription
     Price payable upon exercise of Rights exercised as contemplated by clause
     (a) above, does not exceed the Rights Exercise Commitment Amount of the
     Standby Purchaser as set forth in Annex I hereto; and
                                       -------         

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights").
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     Any Rights that remain unexercised upon expiration thereof will be deemed
     to be "Section 3(b) Rights" up to, but not exceeding, the amount of
     Untracked Rights. The Section 3(b) Rights shall be exercised as follows
     prior to the application of Section 1(c) above and Section 1(c) of the
     Other Standby Purchase Commitments: (A) the Standby Purchaser and the Other
     Standby Purchasers will first be given the opportunity to purchase for cash
     (based on the Subscription Price payable upon exercise of such Rights) the
     Rights Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     designees) certificates representing the Arch Participation Warrants
     contemplated by Section 7 below.

     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.

     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     shares of Existing Arch Common Stock issuable upon conversion of any such
     shares of the Arch Class B Common Stock or exercise of any such Arch
     Participation Warrants (the securities referred to in the foregoing clauses
     (i), (ii) and (iii) are referred to herein as the "Registrable
     Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;

     13.  Section 6.  The proviso at the end of Section 6 of the Standby
          ---------                                                     
Commitment Letter is hereby amended in its entirety to read as follows:

     provided, however, that the Standby Purchaser may not assert the condition
     --------  -------                                                         
     contained in this clause (iii) if each of the Unaffiliated Standby
     Purchasers (other than Credit Suisse First Boston Corporation), acting in
     good faith, shall have waived the condition in clause (iii) of Section 6 of
     the Other Standby Purchase Commitment to which such Unaffiliated Standby
     Purchaser is a party.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h) and 10(i) at the end thereof, with such
Sections 10(g), 10(h) and 10(i) to read in their entirety as follows:

     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may take any action in its capacity as a member of the Official
     Committee of Unsecured Creditors appointed in connection with the Chapter
     11 proceeding (the "Committee") that the Standby Purchaser determines, in
     good faith, after consultation with counsel (which may
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     be counsel to the Committee), is required by its fiduciary duties as a
     member of the Committee under applicable law.

     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) The Standby Purchaser agrees that the waiver by Credit Suisse First
     Boston Corporation of the conditions contained in clause (ii) of Section
     5(a) and clause (ii) of Section 5(k) of the Other Standby Purchase
     Commitment to which it is a party, pursuant to Section 10(i) of such Other
     Standby Purchase Commitment, shall be deemed a good faith waiver for
     purposes of Sections 5(a)(ii) and 5(k)(ii) above.

     17.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".

     18.  New Section 13(c).  Article 13 of the Standby Commitment Letter is
          -----------------                                                 
hereby amended to add the following Section 13(c) of the end thereof:

     (c) Each of the parties hereto acknowledges and agrees that,
     notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may not waive, or otherwise abrogate its right to assert, the
     conditions contained in clause (ii) of Section 5(a), clause (ii) of Section
     5(k) and clause (iii) of Section 6 other than in connection with a
     scheduled Closing.

     19.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 16 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:

     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     20.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     21.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.

     22.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     23.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     24.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.


                              Very truly yours,

                              THE NORTHWESTERN MUTUAL LIFE
                                 INSURANCE COMPANY
                                 FOR ITS GROUP ANNUITY SEPARATE ACCOUNT

                              By:   Northwestern Investment
                                    Management Company

                                    By: /s/
                                       ---------------------------
                                    Name:
                                    Its:

                                    Address:  720 E. Wisconsin Avenue
                                              Milwaukee, WI 53202
                                              Attention: Securities
                                                        Department
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   ----------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer

With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   ----------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>
<PAGE>
 
                     NORTHWESTERN MUTUAL SERIES FUND, INC.
                       for the High Yield Bond Portfolio
                           720 East Wisconsin Avenue
                          Milwaukee, Wisconsin  53202


                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and Northwestern Mutual Series Fund, Inc. for the High
Yield Bond Portfolio.  Terms used herein with initial capital letters that are
not otherwise defined shall have the meanings ascribed to such terms in the
Standby Commitment Letter.

     The parties hereto hereby agree as follows:

     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     of the Debtors in the form attached as Exhibit A to the Merger Agreement,
     with such amendments and modifications thereto as are made in a manner
     consistent with clause (e) of Section 5 hereto (such plan of reorganization
     being referred to herein as the "Plan"), (i) cash and (ii) shares of its
     Common Stock, par value $.01 per share ("Existing Arch Common Stock"); (c)
     holders of unsecured non-priority claims against the Debtors ("Unsecured
     Claims"), to the extent such Unsecured Claims are Allowed (as defined in
     the Plan), will receive pursuant to the Plan (i) an aggregate of 14,344,969
     shares of Existing Arch Common Stock and (ii) rights to purchase ("Rights")
     an aggregate of 108,500,000 shares of Existing Arch Common Stock ("Rights
     Shares") for $2.00 per share in cash (the "Subscription Price"); (d)
     holders of claims arising under or relating to the Credit Agreement, dated
     December 4, 1995, as amended, among MobileMedia and the other parties
     thereto ("Secured Claims"), to the extent such Secured Claims are Allowed,
     will receive pursuant to the Plan cash in an amount equal to 100% of such
     claims; (e) all of the outstanding equity interests in MobileMedia and
     Parent will be canceled without consideration and Parent will be dissolved;
     and (f) the commitments under the DIP Loan Agreement will terminate and all
     amounts owed under or in respect of the DIP Loan Agreement will be paid in
     full in cash. Arch will conduct the Stockholder Rights Offering, in which
     it will issue to holders of Buyer Stock Stockholder Rights to acquire an
     aggregate of 44,893,166 shares of Existing Arch Common Stock, and,
     immediately following the Combination, Arch will issue warrants entitling
     the holders thereof to purchase shares of Existing Arch Common Stock ("Arch
     Participation Warrants"), with such Arch Participation Warrants to be
     issued pursuant to, and to have the terms set forth in, a warrant agreement
     in the form attached as Exhibit B-1 to the Merger Agreement (the "Arch
     Participation Warrant Agreement"), to the stockholders of Arch to the
     extent any Stockholder Rights issued to such Stockholder Rights Holder were
     not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     distributed, in each case as permitted by Section 3 below, and (ii) the
     Rights sold or transferred by the Standby Purchaser or the Rights
     distributed in respect of Unsecured Claims held by it as of the date hereof
     that are hereafter sold or transferred by the Standby Purchaser are not
     exercised prior to the expiration thereof (at which time such Rights will
     be void and will no longer be exercisable), to purchase for cash (based
     upon the Subscription Price payable upon exercise of such Rights) the
     Rights Shares underlying such unexercised Rights, to the extent that the
     aggregate purchase price therefor, together with the aggregate Subscription
     Price payable upon exercise of Rights exercised as contemplated by clause
     (a) above, does not exceed the Rights Exercise Commitment Amount of the
     Standby Purchaser as set forth in Annex I hereto; and
                                       -------        

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights").
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     Any Rights that remain unexercised upon expiration thereof will be deemed
     to be "Section 3(b) Rights" up to, but not exceeding, the amount of
     Untracked Rights. The Section 3(b) Rights shall be exercised as follows
     prior to the application of Section 1(c) above and Section 1(c) of the
     Other Standby Purchase Commitments: (A) the Standby Purchaser and the Other
     Standby Purchasers will first be given the opportunity to purchase for cash
     (based on the Subscription Price payable upon exercise of such Rights) the
     Rights Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------                      
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     designees) certificates representing the Arch Participation Warrants
     contemplated by Section 7 below.

     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.

     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     shares of Existing Arch Common Stock issuable upon conversion of any such
     shares of the Arch Class B Common Stock or exercise of any such Arch
     Participation Warrants (the securities referred to in the foregoing clauses
     (i), (ii) and (iii) are referred to herein as the "Registrable
     Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;

     13.  Section 6.  The proviso at the end of Section 6 of the Standby
          ---------                                                     
Commitment Letter is hereby amended in its entirety to read as follows:

     provided, however, that the Standby Purchaser may not assert the condition
     --------  -------                                                         
     contained in this clause (iii) if each of the Unaffiliated Standby
     Purchasers (other than Credit Suisse First Boston Corporation), acting in
     good faith, shall have waived the condition in clause (iii) of Section 6 of
     the Other Standby Purchase Commitment to which such Unaffiliated Standby
     Purchaser is a party.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h) and 10(i) at the end thereof, with such
Sections 10(g), 10(h) and 10(i) to read in their entirety as follows:

     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may take any action in its capacity as a member of the Official
     Committee of Unsecured Creditors appointed in connection with the Chapter
     11 proceeding (the "Committee") that the Standby Purchaser determines, in
     good faith, after consultation with counsel (which may
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     be counsel to the Committee), is required by its fiduciary duties as a
     member of the Committee under applicable law.

     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) The Standby Purchaser agrees that the waiver by Credit Suisse First
     Boston Corporation of the conditions contained in clause (ii) of Section
     5(a) and clause (ii) of Section 5(k) of the Other Standby Purchase
     Commitment to which it is a party, pursuant to Section 10(i) of such Other
     Standby Purchase Commitment, shall be deemed a good faith waiver for
     purposes of Sections 5(a)(ii) and 5(k)(ii) above.

     17.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".

     18.  New Section 13(c).  Article 13 of the Standby Commitment Letter is
          -----------------                                                 
hereby amended to add the following Section 13(c) of the end thereof:

     (c) Each of the parties hereto acknowledges and agrees that,
     notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may not waive, or otherwise abrogate its right to assert, the
     conditions contained in clause (ii) of Section 5(a), clause (ii) of Section
     5(k) and clause (iii) of Section 6 other than in connection with a
     scheduled Closing.

     19.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 16 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:

     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     20.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     21.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.

     22.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     23.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     24.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.

                              Very truly yours,

                              NORTHWESTERN MUTUAL SERIES FUND, INC.
                                 FOR THE HIGH YIELD BOND PORTFOLIO

                              By: /s/
                                 ---------------------------------
                              Name:
                              Its:

                              Address:   c/o The Northwestern Mutual Life
                                            Insurance Company
                                         720 E. Wisconsin Avenue
                                         Milwaukee, WI 53202
                                         Attention: Securities Department
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   -------------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer
 
With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   -------------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>
<PAGE>
 
                     W.R. HUFF ASSET MANAGEMENT CO., L.L.C.
                           67 Park Place, Ninth Floor
                         Morristown, New Jersey  07960

                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and W.R. Huff Asset Management Co., L.L.C.  Terms used
herein with initial capital letters that are not otherwise defined shall have
the meanings ascribed to such terms in the Standby Commitment Letter.

     The parties hereto hereby agree as follows:

     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization of the Debtors in the form attached as Exhibit A to
     the Merger Agreement, with such amendments and modifications thereto as are
     made in a manner
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     consistent with clause (e) of Section 5 hereto (such plan of reorganization
     being referred to herein as the "Plan"), (i) cash and (ii) shares of its
     Common Stock, par value $.01 per share ("Existing Arch Common Stock"); (c)
     holders of unsecured non-priority claims against the Debtors ("Unsecured
     Claims"), to the extent such Unsecured Claims are Allowed (as defined in
     the Plan), will receive pursuant to the Plan (i) an aggregate of 14,344,969
     shares of Existing Arch Common Stock and (ii) rights to purchase ("Rights")
     an aggregate of 108,500,000 shares of Existing Arch Common Stock ("Rights
     Shares") for $2.00 per share in cash (the "Subscription Price"); (d)
     holders of claims arising under or relating to the Credit Agreement, dated
     December 4, 1995, as amended, among MobileMedia and the other parties
     thereto ("Secured Claims"), to the extent such Secured Claims are Allowed,
     will receive pursuant to the Plan cash in an amount equal to 100% of such
     claims; (e) all of the outstanding equity interests in MobileMedia and
     Parent will be canceled without consideration and Parent will be dissolved;
     and (f) the commitments under the DIP Loan Agreement will terminate and all
     amounts owed under or in respect of the DIP Loan Agreement will be paid in
     full in cash. Arch will conduct the Stockholder Rights Offering, in which
     it will issue to holders of Buyer Stock Stockholder Rights to acquire an
     aggregate of 44,893,166 shares of Existing Arch Common Stock, and,
     immediately following the Combination, Arch will issue warrants entitling
     the holders thereof to purchase shares of Existing Arch Common Stock ("Arch
     Participation Warrants"), with such Arch Participation Warrants to be
     issued pursuant to, and to have the terms set forth in, a warrant agreement
     in the form attached as Exhibit B-1 to the Merger Agreement (the "Arch
     Participation Warrant Agreement"), to the stockholders of Arch to the
     extent any Stockholder Rights issued to such Stockholder Rights Holder were
     not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be distributed, in each case as permitted by Section 3 below,
     and (ii) the Rights sold or transferred by the Standby Purchaser or the
     Rights distributed in respect
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     of Unsecured Claims held by it as of the date hereof that are hereafter
     sold or transferred by the Standby Purchaser are not exercised prior to the
     expiration thereof (at which time such Rights will be void and will no
     longer be exercisable), to purchase for cash (based upon the Subscription
     Price payable upon exercise of such Rights) the Rights Shares underlying
     such unexercised Rights, to the extent that the aggregate purchase price
     therefor, together with the aggregate Subscription Price payable upon
     exercise of Rights exercised as contemplated by clause (a) above, does not
     exceed the Rights Exercise Commitment Amount of the Standby Purchaser as
     set forth in Annex I hereto; and
                  -------        

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights"). Any Rights that remain unexercised
     upon expiration thereof will be deemed to be "Section 3(b) Rights" up to,
     but not exceeding, the amount of Untracked
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     Rights. The Section 3(b) Rights shall be exercised as follows prior to the
     application of Section 1(c) above and Section 1(c) of the Other Standby
     Purchase Commitments: (A) the Standby Purchaser and the Other Standby
     Purchasers will first be given the opportunity to purchase for cash (based
     on the Subscription Price payable upon exercise of such Rights) the Rights
     Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------                      
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its designees) certificates representing the Arch Participation
     Warrants contemplated by Section 7 below.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.

     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all shares of Existing Arch Common Stock
     issuable upon conversion of any such shares of the Arch Class B Common
     Stock or exercise of any such Arch
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     Participation Warrants (the securities referred to in the foregoing clauses
     (i), (ii) and (iii) are referred to herein as the "Registrable
     Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;

     13.  Section 6.  The proviso at the end of Section 6 of the Standby
          ---------                                                     
Commitment Letter is hereby amended in its entirety to read as follows:

     provided, however, that the Standby Purchaser may not assert the condition
     --------  -------                                                         
     contained in this clause (iii) if each of the Unaffiliated Standby
     Purchasers (other than Credit Suisse First Boston Corporation), acting in
     good faith, shall have waived the condition in clause (iii) of Section 6 of
     the Other Standby Purchase Commitment to which such Unaffiliated Standby
     Purchaser is a party.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h) and 10(i) at the end thereof, with such
Sections 10(g), 10(h) and 10(i) to read in their entirety as follows:

     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may take any action in its capacity as a member of the Official
     Committee of Unsecured Creditors appointed in connection with the Chapter
     11 proceeding (the "Committee") that the Standby Purchaser determines, in
     good faith, after consultation with counsel (which may be counsel to the
     Committee), is required by its fiduciary duties as a member of the
     Committee under applicable law.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) The Standby Purchaser agrees that the waiver by Credit Suisse First
     Boston Corporation of the conditions contained in clause (ii) of Section
     5(a) and clause (ii) of Section 5(k) of the Other Standby Purchase
     Commitment to which it is a party, pursuant to Section 10(i) of such Other
     Standby Purchase Commitment, shall be deemed a good faith waiver for
     purposes of Sections 5(a)(ii) and 5(k)(ii) above.

     17.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".

     18.  New Section 13(c).  Article 13 of the Standby Commitment Letter is
          -----------------                                                 
hereby amended to add the following Section 13(c) of the end thereof:

     (c) Each of the parties hereto acknowledges and agrees that,
     notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may not waive, or otherwise abrogate its right to assert, the
     conditions contained in clause (ii) of Section 5(a), clause (ii) of Section
     5(k) and clause (iii) of Section 6 other than in connection with a
     scheduled Closing.

     19.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 17 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:

     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     20.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        

     21.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     22.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     23.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     24.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.

                              Very truly yours,

                              W. R. HUFF ASSET MANAGEMENT CO., L.L.C.


                              By: /s/ Bryan E. Bloom, Esq.
                                 ------------------------------
                              Name: Bryan E. Bloom, Esq.
                              Its:       Attorney-in-Fact

                              Address:   67 Park Place, 9th Floor
                                         Morristown, NJ 07960
                                         Attention: Cathy Markey, Esq.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   -------------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer

With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   -------------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>
<PAGE>
 
                         WHIPPOORWILL ASSOCIATES, INC.
                               11 Martine Avenue
                         White Plains, New York  10606


                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and Whippoorwill Associates, Inc.  Terms used herein with
initial capital letters that are not otherwise defined shall have the meanings
ascribed to such terms in the Standby Commitment Letter.

     The parties hereto hereby agree as follows:

     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization of the Debtors in the form attached as Exhibit A to
     the Merger Agreement,
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     with such amendments and modifications thereto as are made in a manner
     consistent with clause (e) of Section 5 hereto (such plan of reorganization
     being referred to herein as the "Plan"), (i) cash and (ii) shares of its
     Common Stock, par value $.01 per share ("Existing Arch Common Stock"); (c)
     holders of unsecured non-priority claims against the Debtors ("Unsecured
     Claims"), to the extent such Unsecured Claims are Allowed (as defined in
     the Plan), will receive pursuant to the Plan (i) an aggregate of 14,344,969
     shares of Existing Arch Common Stock and (ii) rights to purchase ("Rights")
     an aggregate of 108,500,000 shares of Existing Arch Common Stock ("Rights
     Shares") for $2.00 per share in cash (the "Subscription Price"); (d)
     holders of claims arising under or relating to the Credit Agreement, dated
     December 4, 1995, as amended, among MobileMedia and the other parties
     thereto ("Secured Claims"), to the extent such Secured Claims are Allowed,
     will receive pursuant to the Plan cash in an amount equal to 100% of such
     claims; (e) all of the outstanding equity interests in MobileMedia and
     Parent will be canceled without consideration and Parent will be dissolved;
     and (f) the commitments under the DIP Loan Agreement will terminate and all
     amounts owed under or in respect of the DIP Loan Agreement will be paid in
     full in cash. Arch will conduct the Stockholder Rights Offering, in which
     it will issue to holders of Buyer Stock Stockholder Rights to acquire an
     aggregate of 44,893,166 shares of Existing Arch Common Stock, and,
     immediately following the Combination, Arch will issue warrants entitling
     the holders thereof to purchase shares of Existing Arch Common Stock ("Arch
     Participation Warrants"), with such Arch Participation Warrants to be
     issued pursuant to, and to have the terms set forth in, a warrant agreement
     in the form attached as Exhibit B-1 to the Merger Agreement (the "Arch
     Participation Warrant Agreement"), to the stockholders of Arch to the
     extent any Stockholder Rights issued to such Stockholder Rights Holder were
     not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be distributed, in each case as permitted by Section 3 below,
     and (ii) the Rights
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     sold or transferred by the Standby Purchaser or the Rights distributed in
     respect of Unsecured Claims held by it as of the date hereof that are
     hereafter sold or transferred by the Standby Purchaser are not exercised
     prior to the expiration thereof (at which time such Rights will be void and
     will no longer be exercisable), to purchase for cash (based upon the
     Subscription Price payable upon exercise of such Rights) the Rights Shares
     underlying such unexercised Rights, to the extent that the aggregate
     purchase price therefor, together with the aggregate Subscription Price
     payable upon exercise of Rights exercised as contemplated by clause (a)
     above, does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto; and
                               -------        

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights"). Any Rights that remain unexercised
     upon expiration thereof will be deemed to
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     be "Section 3(b) Rights" up to, but not exceeding, the amount of Untracked
     Rights. The Section 3(b) Rights shall be exercised as follows prior to the
     application of Section 1(c) above and Section 1(c) of the Other Standby
     Purchase Commitments: (A) the Standby Purchaser and the Other Standby
     Purchasers will first be given the opportunity to purchase for cash (based
     on the Subscription Price payable upon exercise of such Rights) the Rights
     Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------                      
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     designees) certificates representing the Arch Participation Warrants
     contemplated by Section 7 below.

     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.

     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     shares of Existing Arch Common Stock issuable upon conversion of any such
     shares of the Arch Class B Common Stock or exercise of any such Arch
     Participation Warrants (the securities referred to in the foregoing clauses
     (i), (ii) and (iii) are referred to herein as the "Registrable
     Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;

     13.  Section 6.  The proviso at the end of Section 6 of the Standby
          ---------                                                     
Commitment Letter is hereby amended in its entirety to read as follows:

     provided, however, that the Standby Purchaser may not assert the condition
     --------  -------                                                         
     contained in this clause (iii) if each of the Unaffiliated Standby
     Purchasers (other than Credit Suisse First Boston Corporation), acting in
     good faith, shall have waived the condition in clause (iii) of Section 6 of
     the Other Standby Purchase Commitment to which such Unaffiliated Standby
     Purchaser is a party.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 9(c).  Section 9(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended to add the following sentence at the end thereof:

     The Whippoorwill Accounts may from time to time agree to reallocate the
     aggregate Unexercised Rights Commitment Amount set forth in Schedule A
     among themselves as may be necessary for the Whippoorwill Accounts to
     achieve compliance with regulatory requirements (including without
     limitation requirements imposed by or related to FCC regulations) and
     diversification requirements applicable to them, and the consent of Arch
     and MobileMedia shall not be required in connection therewith; provided,
                                                                    -------- 
     however, that no reallocation that materially reduces the Unexercised
     -------                                                              
     Rights Commitment Amount of any of the three largest Whippoorwill Accounts
     shall be made without the prior written consent of Arch and MobileMedia,
     which consent shall not be unreasonably withheld, and, provided, however,
                                                            --------  ------- 
     that no such reallocation shall be made if it has the effect of requiring
     additional approvals from the FCC.

     17.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h) and 10(i) at the end thereof, with such
Sections 10(g), 10(h) and 10(i) to read in their entirety as follows:
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).

     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) The Standby Purchaser agrees that the waiver by Credit Suisse First
     Boston Corporation of the conditions contained in clause (ii) of Section
     5(a) and clause (ii) of Section 5(k) of the Other Standby Purchase
     Commitment to which it is a party, pursuant to Section 10(i) of such Other
     Standby Purchase Commitment, shall be deemed a good faith waiver for
     purposes of Sections 5(a)(ii) and 5(k)(ii) above.

     18.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".

     19.  New Section 13(c).  Article 13 of the Standby Commitment Letter is
          -----------------                                                 
hereby amended to add the following Section 13(c) of the end thereof:

     (c) Each of the parties hereto acknowledges and agrees that,
     notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may not waive, or otherwise abrogate its right to assert, the
     conditions contained in clause (ii) of Section 5(a), clause (ii) of Section
     5(k) and clause (iii) of Section 6 other than in connection with a
     scheduled Closing.

     20.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 17 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     21.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        

     22.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.

     23.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     24.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     25.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.

                              Very truly yours,

                              WHIPPOORWILL ASSOCIATES, INC.,
                                 AS GENERAL PARTNER OF AND/OR AGENT FOR,
                                 EACH WHIPPOORWILL ACCOUNT

                              By: /s/ 
                                 ---------------------------------
                              Name:
                              Its:

                              Address:   11 Martine Avenue
                                         White Plains, NY  10606
                                         Attention: David Strumwasser
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   -------------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer

With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   -------------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>
<PAGE>
 
                     CREDIT SUISSE FIRST BOSTON CORPORATION
                        11 Madison Avenue, Fourth Floor
                           New York, New York  10010

                                December 1, 1998

Arch Communications Group, Inc.
1800 West Park Drive
Suite 250
Westborough, Massachusetts  01581

MobileMedia Communications, Inc.
Fort Lee Executive Park
One Executive Drive
Suite 500
Fort Lee, New Jersey  07024

                          Re:  Amendment to Commitment
                               -----------------------

Gentlemen:

     Reference is made to the letter agreement, dated August 18, 1998 (as
amended pursuant to the letter agreement dated September 3, 1998, the "Standby
Commitment Letter"), among Arch Communications Group, Inc., MobileMedia
Communications, Inc. and Credit Suisse First Boston Corporation.  Terms used
herein with initial capital letters that are not otherwise defined shall have
the meanings ascribed to such terms in the Standby Commitment Letter.

     The parties hereto hereby agree as follows:
     1.   First Paragraph.  The second and third sentences of the first
          ---------------                                              
paragraph of the Standby Commitment Letter are hereby amended in their entirety
to read as follows:

     It is our understanding that in connection with the Reorganization, among
     other things:  (a) pursuant to the Agreement and Plan of Merger, dated as
     of August 18, 1998 (as amended by the First Amendment thereto dated as of
     September 3, 1998 and the Second Amendment thereto dated as of December 1,
     1998, the "Merger Agreement"), among Arch, a wholly owned subsidiary of
     Arch ("Merger Sub"), Parent and MobileMedia, MobileMedia will merge with
     and into Merger Sub, with Merger Sub continuing as the surviving
     corporation and a wholly owned subsidiary of Arch; (b) pursuant to the
     Merger Agreement, Arch will make available for distribution pursuant to a
     plan of reorganization of the Debtors in the form attached as Exhibit A to
     the Merger Agreement, with such amendments and modifications thereto as are
     made in a manner consistent with clause (e) of Section 5 hereto (such plan
     of reorganization
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 2


     being referred to herein as the "Plan"), (i) cash and (ii) shares of its
     Common Stock, par value $.01 per share ("Existing Arch Common Stock"); (c)
     holders of unsecured non-priority claims against the Debtors ("Unsecured
     Claims"), to the extent such Unsecured Claims are Allowed (as defined in
     the Plan), will receive pursuant to the Plan (i) an aggregate of 14,344,969
     shares of Existing Arch Common Stock and (ii) rights to purchase ("Rights")
     an aggregate of 108,500,000 shares of Existing Arch Common Stock ("Rights
     Shares") for $2.00 per share in cash (the "Subscription Price"); (d)
     holders of claims arising under or relating to the Credit Agreement, dated
     December 4, 1995, as amended, among MobileMedia and the other parties
     thereto ("Secured Claims"), to the extent such Secured Claims are Allowed,
     will receive pursuant to the Plan cash in an amount equal to 100% of such
     claims; (e) all of the outstanding equity interests in MobileMedia and
     Parent will be canceled without consideration and Parent will be dissolved;
     and (f) the commitments under the DIP Loan Agreement will terminate and all
     amounts owed under or in respect of the DIP Loan Agreement will be paid in
     full in cash. Arch will conduct the Stockholder Rights Offering, in which
     it will issue to holders of Buyer Stock Stockholder Rights to acquire an
     aggregate of 44,893,166 shares of Existing Arch Common Stock, and,
     immediately following the Combination, Arch will issue warrants entitling
     the holders thereof to purchase shares of Existing Arch Common Stock ("Arch
     Participation Warrants"), with such Arch Participation Warrants to be
     issued pursuant to, and to have the terms set forth in, a warrant agreement
     in the form attached as Exhibit B-1 to the Merger Agreement (the "Arch
     Participation Warrant Agreement"), to the stockholders of Arch to the
     extent any Stockholder Rights issued to such Stockholder Rights Holder were
     not exercised.

     2.   Section 1(a).  Section 1(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) to exercise any Rights distributed to it in respect of its Allowed
     Unsecured Claims in accordance with the Plan and not thereafter sold or
     transferred as permitted by Section 3 below to purchase Rights Shares, to
     the extent that the aggregate Subscription Price payable upon such exercise
     does not exceed the Rights Exercise Commitment Amount of the Standby
     Purchaser as set forth in Annex I hereto;
                               -------        

     3.   Section 1(b).  Section 1(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) if (i) the Standby Purchaser sells or otherwise transfers any or all of
     the (A) the Rights distributed to it in accordance with the Plan or (B)
     Unsecured Claims held by it as of the date hereof in respect of which
     Rights are to be distributed, in each case as permitted by Section 3 below,
     and (ii) the Rights sold or transferred by the Standby Purchaser or the
     Rights distributed in respect of Unsecured Claims held by it as of the date
     hereof that are hereafter sold or
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 3


     transferred by the Standby Purchaser are not exercised prior to the
     expiration thereof (at which time such Rights will be void and will no
     longer be exercisable), to purchase for cash (based upon the Subscription
     Price payable upon exercise of such Rights) the Rights Shares underlying
     such unexercised Rights, to the extent that the aggregate purchase price
     therefor, together with the aggregate Subscription Price payable upon
     exercise of Rights exercised as contemplated by clause (a) above, does not
     exceed the Rights Exercise Commitment Amount of the Standby Purchaser as
     set forth in Annex I hereto; and
                  -------        

     4.   Section 1(c).  Section 1(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) if any Rights distributed in accordance with the Plan (other than (i)
     Rights distributed to the Standby Purchaser or the other holders of
     Unsecured Claims listed on Annex I hereto (the "Other Standby Purchasers")
                                -------                                        
     and retained by them (which Rights are referred to in Section 1(a) above
     and Section 1(a) of each of the Other Standby Purchase Commitments (as
     defined in Section 13(a) below)) or (ii) subject to Section 3(b) below,
     Rights distributed in respect of Unsecured Claims held by the Standby
     Purchaser or the Other Standby Purchasers as of the date hereof that are
     hereafter sold or transferred by them (which Rights are referred to in
     Sections 1(b) and 3 hereof and Sections 1(b) and 3 of each of the Other
     Standby Purchase Commitments)) remain unexercised upon the expiration
     thereof (at which time such Rights will be void and will no longer be
     exercisable), to purchase for cash (based on the Subscription Price payable
     upon exercise of such Rights) pro rata in accordance with and up to the
     Unexercised Rights Commitment Amount of the Standby Purchaser as set forth
     in Annex I hereto the Rights Shares underlying such unexercised Rights.
        -------                                                             

     5.   Section 3(b).  Section 3(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Notwithstanding the provisions of clause (Y) of the proviso in Section
     3(a) above, the Standby Purchaser may elect to sell or otherwise transfer
     (i) any or all of the Rights distributed to it in accordance with the Plan
     or (ii) Unsecured Claims in respect of which Rights are to be so
     distributed, in either case without entering into a Tracking Agreement with
     its transferee or transferees (any Rights so transferred and any Rights
     distributed in respect of Allowed Claims so transferred, together with any
     Rights so transferred and any Rights distributed in respect of Allowed
     Claims so transferred by the Other Standby Purchasers pursuant to Section
     3(b) of the Other Standby Purchase Commitments, being referred to herein
     collectively as "Untracked Rights"). Any Rights that remain unexercised
     upon expiration thereof will be deemed to be "Section 3(b) Rights" up to,
     but not exceeding, the amount of Untracked Rights.  The Section 3(b) Rights
     shall be exercised as follows prior to the
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 4


     application of Section 1(c) above and Section 1(c) of the Other Standby
     Purchase Commitments: (A) the Standby Purchaser and the Other Standby
     Purchasers will first be given the opportunity to purchase for cash (based
     on the Subscription Price payable upon exercise of such Rights) the Rights
     Shares underlying a number of unexercised Rights up to the amount of
     Section 3(b) Rights in accordance with the percentages set forth in Column
     D of Annex I hereto and (B) to the extent such Right Shares are not so
          -------
     purchased, the Standby Purchaser and any Other Standby Purchasers that are
     responsible for the existence of the Section 3(b) Rights will be required
     to purchase such Rights Shares pro rata based on the number of Section 3(b)
     Rights resulting from their respective transfers. Nothing in this Section
     3(b) will in any way reduce the commitment of the Standby Purchaser
     specified in Section 1(c) above or the Unexercised Rights Commitment Amount
     as set forth in Annex I hereto.
                     -------        

     6.   Section 4(a).  Section 4(a) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (a) Notwithstanding anything to the contrary herein contained or the terms
     of the Rights or the Plan, subject to the conditions set forth herein, on
     the Effective Date the Standby Purchaser, in satisfaction of the
     Commitment, will deliver at the Closing (i) the aggregate Subscription
     Price payable upon exercise of any Rights exercised by it and (ii) the
     purchase price payable in consideration of any shares of Existing Arch
     Common Stock or, if applicable, Arch Class B Common Stock to be otherwise
     purchased by it pursuant to the Commitment; provided, however, that, if
                                                 --------  -------          
     requested by the Standby Purchaser in writing at least two business days
     prior to the Effective Date, any cash to be distributed to the Standby
     Purchaser in respect of Allowed Secured Claims pursuant to the Plan will,
     prior to the distribution thereof pursuant to the Plan and in accordance
     with the instructions included in such written request, be first applied,
     on behalf of the Standby Purchaser, to the payment of such amounts payable
     on the Effective Date as provided in this Section 4(a).

     7.   Section 4(b).  Section 4(b) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (b) Upon payment of the amounts payable as provided in Section 4(a), on the
     Effective Date at the Closing Arch will deliver to the Standby Purchaser
     (or its designees) certificates representing the shares of Existing Arch
     Common Stock or shares of Arch Class B Common Stock, if applicable, (i)
     issuable upon exercise of any Rights exercised by the Standby Purchaser or
     (ii) otherwise purchased by the Standby Purchaser pursuant to the
     Commitment. At the Closing, Arch will also deliver to the Standby Purchaser
     (or its designees) certificates representing the Arch Participation
     Warrants contemplated by Section 7 below.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 5


     8.   Section 4(e).  Section 4(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser may elect to reduce the number of Rights Shares required to be
     purchased by the Standby Purchaser in satisfaction of its Commitment by a
     number (the "Elected Reduction Number") of Rights Shares equal to or less
     than the product of (i) the number of shares of Existing Arch Common Stock
     issued by Arch in connection with the Stockholder Rights Offering and (ii)
     the percentage in Column D of Annex I hereto specified opposite the Standby
                                   -------                                      
     Purchaser's name (such product being referred to herein as the "Maximum
     Reduction Number").  Subject to the immediately preceding sentence, the
     Standby Purchaser shall determine the Elected Reduction Number in its sole
     discretion; provided, however, that the Standby Purchaser is required to
                 --------  -------                                           
     exercise its right under this Section 4(e) to reduce the number of Rights
     Shares to be purchased by it in satisfaction of its Commitment under
     Section 1(c) above by the lesser of (i) 5,000,000 Rights Shares (or, if the
     Reverse Stock Split (as defined in Section 4(f) below) occurs prior to or
     simultaneously with the Closing, a number of Rights Shares equal to the
     product of (x) 5,000,000 and (y) the Adjustment Fraction (as defined in
     Section 4(f) below)) and (ii) a number of Rights Shares equal to the
     Maximum Reduction Number.

     9.   New Section 4(f).  Section 4 of the Standby Commitment Letter is
          ----------------                                                
hereby amended to add the following Section 4(f) of the end thereof:

     (f) Notwithstanding anything to the contrary herein contained, if the Buyer
     effects the reverse stock split contemplated by Section 4.5 of the Buyer
     Disclosure Schedule (the "Reverse Stock Split") prior to or simultaneously
     the Closing, (i)(A) the number of Plan Shares, (B) the number of Rights
     Shares, and (C) the number of Arch Participation Warrants to be issued
     pursuant to Section 7 below, will be adjusted, in each case, to a number
     equal to the product of (x) the number provided therefor herein and (y) the
     Adjustment Fraction and (ii) the Subscription Price will be adjusted to a
     price equal to the product of (x) $2.00 and (y) the Inverse Adjustment
     Fraction.  For purposes of this Section 4(f) the term "Adjustment Fraction"
     means a fraction, the numerator of which is the total number of shares of
     Buyer Common Stock issued and outstanding immediately following the
     effectiveness of the Reverse Stock Split and the denominator of which is
     the total number of shares of Buyer Common Stock issued and outstanding
     immediately prior to the effectiveness of the Reverse Stock Split, and the
     term "Inverse Adjustment Fraction" means the fraction that is the inverse
     of the Adjustment Fraction.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 6


     10.  Section 5(c).  Section 5(c) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (c) there shall be effective under the Securities Act, a registration
     statement (the "Shelf Registration Statement") covering the resale by the
     Standby Purchaser of (i) all shares of Existing Arch Common Stock and all
     shares of Arch Class B Common Stock, if applicable, (ii) all Arch
     Participation Warrants received by the Standby Purchaser pursuant to this
     letter agreement, and (iii) all shares of Existing Arch Common Stock
     issuable upon conversion of any such shares of the Arch Class B Common
     Stock or exercise of any such Arch Participation Warrants (the securities
     referred to in the foregoing clauses (i), (ii) and (iii) are referred to
     herein as the "Registrable Securities");

     11.  Section 5(e).  Section 5(e) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (e) any and all amendments or modifications to the Merger Agreement or any
     exhibit or schedule thereto (including without limitation the Plan, the
     Arch Charter Amendment, the Arch Participation Warrant Agreement and the
     Registration Rights Agreement) on or after the date hereof and any consents
     or waivers delivered on or after the date hereof by Arch or MobileMedia to
     the other under the Merger Agreement (other than (i) subject to Section
     15(a) below, consents under Section 4.5 of the Merger Agreement, (ii)
     waivers of Unilateral Conditions or (iii) any amendment to the Merger
     Agreement solely to reduce the amount of the Buyer Breakup Fee) shall have
     been in form and substance reasonably satisfactory to the Standby
     Purchaser;

     12.  Section 5(j).  Section 5(j) of the Standby Commitment Letter is hereby
          ------------                                                          
amended in its entirety to read as follows:

     (j) (i) the shares of Existing Arch Common Stock to be issued and
     distributed as contemplated by Section 1.3(e) and Section 1.6 of the Merger
     Agreement shall be so issued and distributed pursuant to an exemption from
     registration under the Securities Act provided by Section 1145 of the
     Bankruptcy Code, and (ii)(A) the issuance of the Rights, (B) the issuance
     of the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, upon exercise of the Rights, (C) the issuance
     to the Standby Purchaser of the shares of Existing Arch Common Stock and
     the shares of Arch Class B Common Stock, if applicable, as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants as
     contemplated by Section 7 below, and (D) the issuance of Existing Arch
     Common Stock upon exercise of the Arch Participation Warrants or conversion
     of Arch Class B Common Stock, if applicable, shall be covered by the
     Registration Statement, the Registration Statement shall have been declared
     effective and no stop order with respect thereto shall be in effect;
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 7


     13.  Section 6.  Section 6 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     6.  [Intentionally omitted]

     14.  Section 7.  Section 7 of the Standby Commitment Letter is hereby
          ---------                                                       
amended in its entirety to read as follows:

     7.  Consideration for the Commitment.  In consideration for the Commitment,
         --------------------------------                                       
     on the Effective Date at the Closing the Standby Purchaser will receive the
     number of Arch Participation Warrants specified in Column E of Annex I
                                                                    -------
     hereto.

     15.  Section 9(a)(x).  Section 9(a)(x) of the Standby Commitment Letter is
          ---------------                                                      
hereby amended in its entirety to read as follows:

     (x) The shares of Existing Arch Common Stock to be issued and distributed
     as contemplated by Section 1.3(e) and Section 1.6 of the Merger Agreement
     and the shares of Existing Arch Common Stock and the shares of Arch Class B
     Common Stock, if applicable, to be issued and delivered as contemplated by
     Section 1 and Section 3 above and the Arch Participation Warrants to be
     issued as contemplated by Section 7 above, in each case, when so issued and
     distributed or delivered, as the case may be, and the shares of Existing
     Arch Common Stock issued upon conversation of such shares of Arch Class B
     Common Stock, if applicable, when so converted in accordance with the Arch
     Charter Amendment, and the shares of Existing Arch Common Stock issued upon
     exercise of Arch Participation Warrants, when issued, paid for and
     delivered as provided in the Arch Participation Warrant Agreement, will be
     duly authorized, validly issued, fully paid, nonassessable and free of all
     preemptive rights; and

     16.  Section 10.  Section 10 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to add Sections 10(g), 10(h), 10(i) and 10(j) at the end thereof, with
such Sections 10(g), 10(h), 10(i) and 10(j) to read in their entirety as
follows:

     (g) The Standby Purchaser will not, and will cause each of its respective
     directors, officers, employees, representatives and agents not to, directly
     or indirectly, solicit, initiate, engage or participate in or encourage
     discussions or negotiations with any person or entity concerning any
     Acquisition Proposals (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
     The Standby Purchaser will immediately cease any and all existing
     activities, discussions or negotiations with any person with respect to any
     Acquisition Proposal (other than with Arch or the Debtors concerning the
     transactions contemplated hereby, including any modifications thereto).
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 8


     (h) Without intending to limit the generality or effect of Section 10(g)
     above, the Standby Purchaser expressly covenants that it will not enter
     into any agreement, arrangement or understanding (other than those set
     forth in this letter agreement) with any party pursuant to which the
     Standby Purchaser would,  directly or indirectly, provide any financing in
     connection with any Acquisition Proposal or any plan of reorganization of
     the Debtors, whether filed by the Debtors or any other person; provided,
                                                                    -------- 
     however, that nothing herein contained will prevent the Standby Purchaser
     -------                                                                  
     from accepting its pro rata portion of any distribution made to creditors
     of the Debtors in connection with any such Acquisition Proposal or plan of
     reorganization or from exercising any rights, warrants or options received
     by the Standby Purchaser as part of such a pro rata distribution.

     (i) Notwithstanding anything to the contrary herein contained, the Standby
     Purchaser will be deemed to have waived (i) the condition contained in
     clause (ii) of Section 5(a) above except in the event that any of the
     Unaffiliated Standby Purchasers, other than in connection with a scheduled
     Closing, has waived or otherwise abrogated its right to assert the
     condition contained in clause (ii) of Section 5(a) of the Other Standby
     Purchase Commitment to which such Unaffiliated Standby Purchaser is a party
     and (ii) the condition contained in clause (ii) of Section 5(k) above
     except in the event that any of the Unaffiliated Standby Purchasers, other
     than in connection with a scheduled Closing, has waived or otherwise
     abrogated its right to assert the condition contained in clause (ii) of
     Section 5(k) of the Other Standby Purchase Commitment to which such
     Unaffiliated Standby Purchaser is a party.

     (j) As promptly as practicable following the approval of the Disclosure
     Statement by the Bankruptcy Court, the Standby Purchaser will (a) deposit
     or cause to be deposited with a financial institution reasonably acceptable
     to Arch and the Standby Purchaser (the "Escrow Agent") $2,000,000 in cash,
     to be held by the Escrow Agent for disbursement in accordance with an
     escrow agreement to be negotiated in good faith by Arch and the Standby
     Purchaser, (b) obtain a $2,000,000 letter of credit issued by a financial
     institution reasonably acceptable to Arch (which may be an affiliate of the
     Standby Purchaser) having terms reasonably satisfactory to the Standby
     Purchaser and Arch, or (c) negotiate in good faith such alternative
     arrangements as shall be mutually agreeable between the Standby Purchaser
     and Arch; provided, however, that the Standby Purchaser and Arch may
               --------  -------                                         
     mutually agree that there is no need for such deposit, letter of credit or
     alternative arrangement, in which case this Section 10(j) will have no
     further force or effect.

     17.  Section 12.  Section 12 of the Standby Commitment Letter is hereby
          ----------                                                        
amended to delete the phrase "and the Confirmation Order is entered not later
than March 31, 1999".
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 9


     18.  Conclusion.  The third sentence of the paragraph immediately following
          ----------                                                            
Section 16 of the Standby Commitment Letter is hereby amended to read in its
entirety as follows:

     Once effective, this letter agreement will terminate on (i) June 30, 1999,
     unless the effectiveness of the Plan occurs on or before such date, or (ii)
     if not theretofore terminated pursuant to the foregoing clause, the date on
     which the Merger Agreement is terminated in accordance with the terms
     thereof.

     19.  Annex I.  Annex I to the Standby Commitment Letter (as heretofore
          -------                                                          
amended) is hereby further amended to read in its entirety as Annex I hereto.
                                                              -------        

     20.  Continuation of Standby Commitment Letter.  Except as specifically
          -----------------------------------------                         
amended hereby, the Standby Commitment Letter shall continue in full force and
effect and is hereby certified and confirmed in all respects.

     21.  Consent to Amendments.  The Standby Purchaser hereby (a) consents to
          ---------------------                                               
the amendments to the Other Standby Purchase Commitments to be effected by the
letter agreements attached as Exhibit C-2 hereto, which amendments shall be
                              -----------                                  
entered into simultaneously herewith, (b) consents to (i) the Second Amendment
dated as of December 1, 1998 to the Merger Agreement and each of the exhibits,
schedules and annexes attached thereto and (ii) the Third Amended Joint Plan of
Reorganization dated as of December 1, 1998 and each of the exhibits, schedules
and annexes attached thereto, and (c) agrees that the form and substance thereof
are reasonably satisfactory to the Standby Purchaser.

     22.  Governing Law.  This letter agreement shall be governed by and
          -------------                                                 
construed in accordance with the laws of the State of Delaware without regard to
its principles of conflicts of law.

     23.  Counterparts.  This letter agreement may be executed in counterparts
          ------------                                                        
which, taken together, shall constitute one and the same instrument.
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 10


     If the foregoing accurately reflects your understanding with respect to the
matters set forth herein, please confirm by executing and returning a copy of
this letter to the undersigned, whereupon this letter will become a valid and
binding obligation of each party hereto.

                              Very truly yours,

                              CREDIT SUISSE FIRST BOSTON CORPORATION

                              By: /s/
                                 -------------------------------
                              Name:
                              Its:

                              Address:   11 Madison Avenue
                                         New York, NY  10010
                                         Attention: David J. Matlin
                                                    Alex Lagetko

                              With a copy to:

                              Cadwalader, Wickersham & Taft
                              100 Maiden Lane
                              New York, NY  10038
                              Attn:  Michael J. Sage
<PAGE>
 
Arch Communications Group, Inc.
MobileMedia Communications, Inc.
Page 11


ACCEPTED AND AGREED TO:

Arch Communications Group, Inc.

By: /s/ J. Roy Pottle
   -------------------------------
Name:  J. Roy Pottle
Its:  Executive Vice President and
         Chief Financial Officer

Address:  1800 West Park Drive, Suite 250
          Westborough, MA  01581
          Attn:  Chairman and Chief
                 Executive Officer

With a copy to:  Hale and Dorr LLP
                 60 State Street
                 Boston, MA  02109
                 Attn:  Jay E. Bothwick


Subject to entry of the Confirmation Order:

MobileMedia Communications, Inc.

By: /s/ Joseph A. Bondi
   -------------------------------
Name:     Joseph A. Bondi
Its:  Chairman-Restructuring

Address:  Fort Lee Executive Park
          One Executive Drive, Suite 500
          Fort Lee, NJ  07024
          Attn:  Chairman-Restructuring

With a copy to:  Sidley & Austin
                 875 Third Avenue, Suite 1400
                 New York, NY  10022
                 Attn:  James D. Johnson
<PAGE>
 
                                                                         ANNEX I
                                                                         -------

<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
W.R. Huff Asset Management Co., L.L.C.,          $ 39.27          $ 35.80      $ 75.07      34.60%       1,704,006
 as agent for its affiliates and
 discretionary accounts
67 Park Place, 9th Floor
Morristown, New Jersey  07960
Stated Principal Amount of 9 3/8% Notes:
$   57,847,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   27,970,000
----------------

The Northwestern Mutual Life Insurance           $ 10.95          $  9.97      $ 20.92       9.64%         474,861
 Company/1/
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$   19,776,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$    3,350,000
----------------  

The Northwestern Mutual Life Insurance           $  2.65          $  2.42      $  5.07       2.34%/1/      115,084
 Company for its Group Annuity
 Separate Account*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2% Notes
$    7,000,000
----------------  
</TABLE> 
<PAGE>
 
<TABLE>
<CAPTION>
                                                COMMITMENT AMOUNTS
                                                ------------------
                                               (dollars in millions)

                                                 Column A        Column B     Column C    Column D      Column E
                                             ----------------  ------------  ----------  -----------  -------------

                                                               Unexercised                                          
                                             Rights Exercise     Rights        Total                                
                                                Commitment     Commitment    Commitment  Commitment   Participation 
Name and Address of Standby Purchasers          Amount (1)      Amount (2)     Amount    Percentage     Warrants   
--------------------------------------       ---------------   -----------   ----------  ----------   ------------- 
<S>                                          <C>               <C>           <C>         <C>          <C>
Northwestern Mutual Series Fund, Inc. for        $   .75          $   .69      $  1.44       0.66%          32,686
 the High Yield Bond Portfolio*
720 East Wisconsin Avenue
Milwaukee, Wisconsin  53202
Stated Principal Amount of 9 3/8% Notes:
$          -0-
----------------  
Stated Principal Amount of 10 1/2%
 Notes:
$     2,000,000
----------------  

Credit Suisse First Boston Corporation           $ 29.48          $ 26.88      $ 56.36      25.97%          29,309
11 Madison Avenue, 4th Floor
New York, New York  10010
Stated Principal Amount of 9 3/8% Notes:
$   32,453,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   35,930,000
----------------  

Whippoorwill Associates, Inc., as general        $ 30.42          $ 27.72      $ 58.14      26.79%       1,319,713
 partner and/or agent for the parties set
 forth on Schedule A hereto in the
 percentages noted thereon
11 Martine Avenue
White Plains, New York 10606
Stated Principal Amount of 9 3/8% Notes:
$   37,855,000
----------------  
Stated Principal Amount of 10 1/2% Notes:
$   31,410,000
----------------  
                                   Total:        $113.52          $103.48      $217.00     100.00%       3,675,659
                                                 -------          -------      -------                   ---------
</TABLE>
----------------
*  The Northwestern Mutual Life Insurance Company, The Northwestern Mutual Life
Insurance Company for its Group Annuity Separate Account and Northwestern Mutual
Series Fund, Inc. for the High Yield Bond Portfolio are affiliated entities for
purposes of clauses (a) and (k) of Section 5 and clause (iii) of Section 6.

(1)The "Rights Exercise Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to the product of (i) the Subscription Price (which is
subject to adjustment as provided in Section 4(f) of the Standby Commitment
Letter) and (ii) the number of Rights issuable in respect of an amount of
Allowed Unsecured Claims derived from the principal amount of 9 3/8% Notes and
the 10 1/2% Notes indicated under the Standby Purchaser's name on this Annex I
held by such Standby Purchaser on the date hereof.  The dollar amounts set forth
under Column A are estimates provided for illustrative purposes only, based on
the assumptions that (x) there is a total of $475 million of Allowed Unsecured
Claims, (y) there is no Rights Reserve (as defined in the Plan) and (z) the
aggregate amount of Subordinated Noteholder Claims (as defined in the Plan) is
$441,819,762.

(2)The "Unexercised Rights Commitment Amount", for each Standby Purchaser, shall
mean an amount equal to (i) the Total Commitment Amount indicated in Column C
for such Standby Purchaser less (ii) the Rights Exercise Commitment Amount for
such Standby Purchaser.  The dollar amounts set forth under Column B are
estimates provided for illustrative purposes only, based on the estimates set
forth in Column A.
<PAGE>
 
                                                                      SCHEDULE A
                                                                      ----------
<TABLE>
<CAPTION>
                                                       (DOLLARS IN MILLIONS)
                                                         PERCENTAGE OF                        UNEXERCISED
                                                           TOTAL FACE        RIGHTS EXERCISE    RIGHTS         TOTAL
                                          FACE AMOUNT       AMOUNT OF          COMMITMENT     COMMITMENT    COMMITMENT    COMMITMENT
                ACCOUNT                    OF CLAIM           CLAIM              AMOUNT         AMOUNT        AMOUNT      PERCENTAGE
----------------------------------------  -----------  --------------------  ---------------  -----------  -------------  ----------
<S>                                       <C>          <C>                   <C>              <C>          <C>            <C>
President & Fellows of Harvard College      $18.60            26.85%              $ 8.12         $ 4.91       $13.03        22.41%
The Rockefeller Foundation                    6.79             9.81%                2.95           3.85         6.80        11.69%
Vega Partners II, L.P.                        7.84            11.32%                3.40           4.25         7.65        13.15%
Vega Partners III, L.P.                      20.47            29.55%                8.95           3.97        12.92        22.22%
Vega Partners IV, L.P.                        5.52             7.97%                2.43           8.89        11.33        19.48%
Vega Offshore Fund Trust                      9.99            14.42%                4.53           1.86         6.39        10.99%
The Whippoorwill Profit Sharing Plan          0.06             0.09%                0.03              -         0.03         0.05%
                                            ------                                ------         ------       ------       
TOTAL                                       $69.27           100.00%              $30.42         $27.72       $58.14       100.00%
                                            ======                                ======         ======       ======       
</TABLE>