<PAGE>   1
                                                                 EXHIBIT NO. 4.5


                          REGISTRATION RIGHTS AGREEMENT

                                 April 11, 2001

To each of the persons
named on Schedule I hereto:

Dear Sirs:

         This will confirm that in consideration of the consummation of the
merger of Boston Institutional Group, Inc. ("BIG") with and into The BISYS
Group, Inc. (the "Company") pursuant to the Agreement and Plan of Merger dated
as of February 9, 2001 among the Company, BIG and the other persons named
therein, as amended by Amendment No. 1 thereto dated as of April 6, 2001 (the
"Merger Agreement"), in which an aggregate number of shares of common stock,
$.02 par value, of the Company (the "Shares") set forth on Schedule I hereto
opposite your name will be issuable to you upon conversion of shares of common
stock of, or other equity interests in, BIG, the Company hereby covenants and
agrees with each of you, as follows:

         1. Certain Definitions. As used herein, the following terms shall have
the following respective meanings:

                  "Commission" shall mean the United States Securities and
         Exchange Commission, or any other federal agency at the time
         administering the Securities Act (hereinafter defined).

                  "Common Stock" shall mean the common stock, $.02 par value, of
         the Company.

                  "Distribution Period" shall mean the period commencing on the
         effective date of the registration statement referred to in Sections 2
         and 3 hereof and ending on the earlier to occur of (i) the sale of all
         of the Registerable Stock (hereinafter defined) covered by such
         registration statement or (ii) the first anniversary of the Effective
         Time.

                  "Effective Time" shall mean the Effective Time of the Merger,
         as defined in the Merger Agreement.

                  "Exchange Act" shall mean the Securities Exchange Act of 1934
         or any similar federal statute, and the
<PAGE>   2
         rules and regulations of the Commission thereunder, all as the same
         shall be in effect at the time.

                  "Merger" shall mean the merger of BIG with and into the
         Company pursuant to the Merger Agreement.

                  "Registerable Stock" shall mean shares of Common Stock issued
         to you in the Merger and owned by you or a person or entity specified
         in clause (ii), (iii) or (iv) of Section 8(a) of this Agreement,
         provided that such shares of Common Stock shall cease to be
         Registerable Stock when sold pursuant to a registration statement filed
         pursuant to the Agreement.

                  "Securities Act" shall mean the Securities Act of 1933, as
         amended, or any similar federal statute, and the rules and regulations
         of the Commission thereunder, all as the same shall be in effect at the
         time.

         2. Registration of Registerable Stock.

                  Promptly after the Effective Time, the Company shall use its
         reasonable best efforts to register under the Securities Act for public
         resale all shares of Registerable Stock. The Company shall not be
         obligated to effect registration of Registerable Stock pursuant to this
         Section 2 on more than one occasion.

         3. Registration Procedures. The Company will, as expeditiously as is
practicable after the Effective Time:

                  (a) prepare, submit to you and to one counsel for the sellers
         of Registerable Stock, for a reasonable opportunity to review, and
         thereafter file with the Commission as soon as practicable, a
         registration statement with respect to such Registerable Stock (which
         shall be on Form S-3 if the Company is then eligible to use such form
         and otherwise on Form S-1 or such other form of general applicability
         acceptable to the Company), and shall use its reasonable best efforts
         to cause such registration statement to become and remain continuously
         effective during the Distribution Period.

                  (b) prepare and file with the Commission such amendments and
         supplements to such registration statement and the prospectus used in
         connection therewith as may be necessary to keep such registration
         statement continuously effective during the Distribution Period.

                                       2
<PAGE>   3
                  (c) furnish to each seller of Registerable Stock such number
         of copies of the registration statement and the prospectus included
         therein and any amendment or supplement thereto as such person may
         reasonably request in order to facilitate the public sale of the
         Registerable Stock covered by such registration statement;

                  (d) use its best efforts to register or qualify the
         Registerable Stock covered by such registration statement under the
         securities or blue sky laws of a reasonable number of jurisdictions
         (provided that the Company will not be required to (i) qualify
         generally to do business in any jurisdiction where it would not
         otherwise be required to qualify but for this paragraph (d), (ii)
         subject itself to taxation in any such jurisdiction or (iii) consent to
         general service of process in any jurisdiction);

                  (e) promptly notify each seller of Registerable Stock (i) when
         such registration statement or any amendment or supplement thereto or
         to the prospectus contained therein has been filed, (ii) of any request
         by the Commission for amendments or supplements to such registration
         statement or prospectus or for additional information from such seller,
         (iii) of the issuance by the Commission of any stop order suspending
         the effectiveness of such registration statement or the initiation of
         proceedings for that purpose, or (iv) of the receipt by the Company of
         any notification with respect to the suspension of the qualification of
         the Registerable Stock for sale in any jurisdiction or the initiation
         or threatening of any proceeding for that purpose;

                  (f) promptly notify each seller of Registerable Stock at any
         time when a prospectus relating thereto is required to be delivered
         under the Securities Act, of the happening of any event of which the
         Company is aware as a result of which the prospectus contained in such
         registration statement, as then in effect, includes an untrue statement
         of a material fact or omits to state any material fact required to be
         stated therein or necessary to make the statements therein not
         misleading in the light of the circumstances then existing, and as
         promptly as practicable thereafter prepare and file with the Commission
         a supplement or amendment to such prospectus, such registration
         statement or any document incorporated therein by


                                       3
<PAGE>   4
         reference, or make such other filing, such that as thereafter delivered
         to the purchasers of Registerable Stock the prospectus will not contain
         an untrue statement of material fact or omit to state any material fact
         necessary to make the statements therein not misleading provided,
         however, in no event shall the registration statement be unavailable
         (i) for any continuous period in excess of sixty (60) days or (ii) for
         an aggregate of ninety (90) days or more, during the Distribution
         Period.

                  (g) use its reasonable best efforts to obtain the withdrawal
         of any order suspending the effectiveness of such registration
         statement.

         In connection with each registration hereunder, each seller of
Registerable Stock shall furnish to the Company in writing such information with
respect to himself and the proposed distribution by him as shall be reasonably
necessary in order to assure compliance with federal and applicable state
securities laws, including, without limitation, such information as shall enable
the Company to prepare prospectus supplements as and when required to enable
pledgees or donees of such seller to sell shares of Registerable Stock under the
registration statement filed pursuant to Section 2 hereof.

         4. Expenses. All expenses incurred by the Company in complying with
Sections 2 and 3 hereof, including, without limitation, all registration and
filing fees, printing expenses, fees and disbursements of counsel and
independent public accountants for the Company, fees of the National Association
of Securities Dealers, Inc., transfer taxes, fees of transfer agents and
registrars, and the reasonable fees and disbursements of one counsel for the
holders of Registerable Stock will be paid by the Company in connection with the
registration statement filed pursuant to Sections 2 and 3 hereof.

         5. Indemnification. The Company will indemnify and hold harmless each
seller of Registerable Stock under the registration statement filed pursuant to
Sections 2 and 3 and each other person, if any, who controls such seller within
the meaning of the Securities Act, and each partner, director or officer of such
persons, against any losses, claims, damages or liabilities, joint or several,
to which such seller, partner, officer, director or controlling person may
become subject under the Securities Act or otherwise, insofar as such losses,
claims, damages or liabilities (or actions in respect thereof) arise out of or
are based upon (i) any untrue statement or alleged untrue statement of any
material fact contained or incorporated by reference in the registration
statement filed pursuant to Sections 2 and 3 hereof,

                                       4
<PAGE>   5
any prospectus contained therein, or any amendment or supplement thereof, (ii)
the omission or alleged omission to state therein a material fact required to be
stated therein or necessary to make the statements therein not misleading, and
(iii) any violation or alleged violation by the Company of the Securities Act,
the Exchange Act, any state securities law or any rule or regulation promulgated
under any of the same in connection with the offering covered by such
registration statement, and the Company will reimburse each such seller,
partner, officer, director and each such controlling person for any legal or
other expenses reasonably incurred by them in connection with investigating or
defending any such loss, claim, damage, liability or action; provided, however,
that the Company will not be liable in any such case if and to the extent that
any such loss, claim, damage or liability arises out of or is based upon an
untrue statement or alleged untrue statement or omission or alleged omission so
made in reliance upon and in conformity with information furnished by such
seller or any of its partners, officers, directors or such controlling person in
writing specifically for use in such registration statement or prospectus.

                     Each seller of such Registerable Stock under the
registration statement filed pursuant to Sections 2 and 3 hereof, severally and
not jointly, will indemnify and hold harmless the Company and each person, if
any, who controls the Company within the meaning of the Securities Act, each
officer of the Company who signs the registration statement, each director of
the Company, and each person who controls any of the foregoing within the
meaning of the Securities Act, against all losses, claims, damages or
liabilities, joint or several, to which the Company or such officer or director
or controlling person may become subject under the Securities Act or otherwise,
insofar as such losses, claims, damages or liabilities (or actions in respect
thereof) arise out of or are based upon (i) any untrue statement or alleged
untrue statement of any material fact contained in the registration statement
filed pursuant to Sections 2 and 3 hereof, any prospectus contained therein, or
any amendment or supplement thereof, (ii) the omission or alleged omission to
state therein a material fact required to be stated therein or necessary to make
the statements therein not misleading, or (iii) any violation or alleged
violation by the Company of the Securities Act, the Exchange Act, any state
securities law or any rule or regulation promulgated under any of the same in
connection with the offering covered by such registration statement, and each
such seller will reimburse the Company and each such officer, director and
controlling person for any legal or other expenses reasonably incurred by them
in connection with investigating or defending any such loss, claim, damage,
liability or action; provided, however, that such seller will be liable
hereunder in any such case if and only to the extent that any such loss, claim,
damage or liability


                                       5
<PAGE>   6
arises out of or is based upon an untrue statement or alleged untrue statement
or omission or alleged omission made in reliance upon and in conformity with
information furnished in writing to the Company by such seller specifically for
use in such registration statement or prospectus. Notwithstanding the foregoing,
(i) no seller shall be liable for payments of amounts paid in settlement of any
loss, claim, damage, liability or action if such settlement is effected without
the consent of such seller (which consent shall not be unreasonably withheld),
and (ii) in no event shall the liability of any seller of Registerable Stock
under this Section 5 in connection with any registration exceed the proceeds
received by such seller from the sale of shares of Registerable Stock in such
registration.

         Promptly after receipt by an indemnified party hereunder of notice of
the commencement of any action, such indemnified party shall, if a claim in
respect thereof is to be made against the indemnifying party hereunder, notify
the indemnifying party in writing thereof; provided, however, that the failure
of any indemnified party to give notice to such indemnifying party as provided
herein shall not relieve such indemnifying party of its obligations under the
foregoing provisions of this Section 5, except and solely to the extent that
such indemnifying party is actually prejudiced by such failure to give notice.
In case any such action shall be brought against any indemnified party and it
shall notify the indemnifying party of the commencement thereof, the
indemnifying party shall be entitled to participate in and, to the extent it
shall wish, to assume and undertake the defense thereof with counsel
satisfactory to such indemnified party, and, after notice from the indemnifying
party to such indemnified party of its election so to assume and undertake the
defense thereof, the indemnifying party, if it shall actually undertake the
defense thereof, shall not be liable to such indemnified party under this
Section 5 for any legal expenses subsequently incurred by such indemnified party
in connection with the defense thereof other than reasonable costs of
investigation and of liaison with counsel so selected; provided, however, that,
if the defendants in any such action include both the indemnified party and the
indemnifying party and the indemnified party shall have reasonably concluded
that there may be reasonable defenses available to it which are different from
or additional to those available to the indemnifying party, or if the interests
of the indemnified party reasonably may be deemed to conflict with the interests
of the indemnifying party, the indemnified party shall have the right to select
a separate counsel and to assume such legal defenses and otherwise to
participate in the defense of such action, with the expenses and fees of such
separate counsel and other expenses related to such participation to be
reimbursed by the indemnifying party as incurred.

                                       6
<PAGE>   7
         Any indemnified party shall have the right to retain its own counsel in
any such action, but the fees and disbursements of such counsel shall be at the
expense of such indemnified party unless (i) the indemnifying party shall have
failed to retain counsel for the indemnified person as aforesaid or shall have
failed to defend such action in accordance with the preceding paragraph or (ii)
the indemnifying party and such indemnified party shall have mutually agreed to
the retention of such counsel. It is understood that the indemnifying party
shall not, in connection with any action or related actions in the same
jurisdiction, be liable for the fees and disbursements of more than one separate
firm qualified in such jurisdiction to act as counsel for the indemnified party.
The indemnifying Party shall not, without the consent of the indemnified Party,
which consent shall not be unreasonably withheld or delayed, consent to the
entry of any judgment or enter into any settlement with respect to such action;
provided, that, unless agreed to by the indemnified party, such judgment or
settlement must include an unconditional release of the indemnified party from
all liability in respect of such action. The indemnifying party shall not be
liable for any settlement of any proceeding effected without its written
consent, but if settled with such consent or if there be a final judgment for
the plaintiff, the indemnifying party agrees to indemnify the indemnified party
from and against any loss or liability by reason of such settlement or judgment.

         If the indemnification provided for in the first two paragraphs of this
Section 5 is unavailable or insufficient to hold harmless an indemnified party
under such paragraphs in respect of any losses, claims, damages or liabilities
or actions in respect thereof referred to therein, then each indemnifying party
shall in lieu of indemnifying such indemnified party contribute to the amount
paid or payable by such indemnified party as a result of such losses, claims,
damages, liabilities or actions in such proportion as appropriate to reflect the
relative fault of the Company, on the one hand, and the sellers of such
Registerable Stock, on the other, in connection with the statements or omissions
which resulted in such losses, claims, damages, liabilities or actions as well
as any other relevant equitable considerations. The relative fault shall be
determined by reference to, among other things, whether the untrue or alleged
untrue statement of a material fact relates to information supplied by the
Company, on the one hand, or the sellers of such Registerable Stock, on the
other, and to the parties' relative intent, knowledge, access to information and
opportunity to correct or prevent such statement or omission. The Company and
each of you agree that it would not be just and equitable if contributions
pursuant to this paragraph were determined by pro rata allocation (even if all
of the sellers of such Registerable Stock were treated as one entity for such
purpose) or by any other


                                       7
<PAGE>   8
method of allocation which did not take account of the equitable considerations
referred to above in this paragraph. The amount paid or payable by an
indemnified party as a result of the losses, claims, damages, liabilities or
action in respect thereof, referred to above in this paragraph, shall be deemed
to include any legal or other expenses reasonably incurred by such indemnified
party in connection with investigating or defending any such action or claim.
Notwithstanding the provisions of this paragraph, the sellers of such
Registerable Stock shall not be required to contribute any amount in excess of
the amount, if any, by which the total price at which the Common Stock sold by
each of them was offered to the public exceeds the amount of any damages which
they would have otherwise been required to pay by reason of such untrue or
alleged untrue statement or omission. No person guilty of fraudulent
misrepresentations (within the meaning of Section 11(f) of the Securities Act),
shall be entitled to contribution from any person who is not guilty of such
fraudulent misrepresentation.

         6. Current Public Information. The Company agrees with you as follows:

                  (a) The Company shall use its reasonable best efforts to make
         and keep public information available, as those terms are understood
         and defined in Rule 144 under the Securities Act, at all times from and
         after the date hereof.

                  (b) The Company shall use its reasonable best efforts to file
         with the Commission in a timely manner all reports and other documents
         as the Commission may prescribe under Section 13(a) or 15(d) of the
         Exchange Act.

                  (c) The Company shall furnish to each seller of Registerable
         Stock upon request (i) a written statement by the Company as to its
         compliance with the reporting requirements under the Exchange Act, as
         required for the use of Rule 144 under the Securities Act, (ii) a copy
         of the most recent annual or quarterly report of the Company, and (iii)
         such other reports and documents so filed as a holder may reasonably
         request to avail itself of any rule or regulation of the Commission
         allowing a holder of Registerable Stock to sell any such securities
         without registration.

         7. Effectiveness of this Agreement. This Agreement shall become
effective at the Effective Time. If the Effective Time shall not occur, this
Agreement shall be of no force and effect.

                                       8
<PAGE>   9
         8. Miscellaneous.

         (a) All covenants and agreements contained in this Agreement by or on
behalf of any of the parties hereto shall bind and inure to the benefit of the
respective successors and assigns of the parties hereto whether so expressed or
not, provided, however, that the obligations of the Company hereunder shall
inure only to the benefit of (i) you, (ii) a person who shall become a holder of
Registerable Stock by gift or by will or the laws of descent and distribution,
(iii) an institutional lender to whom shares of Registerable Stock are bona fide
pledged as collateral security for a loan or (iv) any entity in which you and
members of your immediate family own in excess of 50% of the equity or
beneficial ownership, and the term "Registerable Stock" as used herein shall be
limited to Registerable Stock held by you or any such person or entity.

         (b) All notices, requests, consents and other communications hereunder
shall be in writing and shall be mailed by first class registered mail, postage
prepaid, addressed as follows:

                  if to the Company, to it at Overlook at Great Notch, 150 Clove
         Road, Little Falls, New Jersey 07424, Attention: Executive Vice
         President and General Counsel;


         if to any holder of Registerable Stock, at its address as set forth in
         Annex I hereto;

                  if to any subsequent holder of Registerable Stock pursuant to
         Section 8(a) hereof to it at such address as may have been furnished to
         the Company in writing by such holder;

         or, in any case, at such other address or addresses as shall have been
         furnished in writing to the Company (in the case of a seller of
         Registerable Stock) or to the sellers of Registerable Stock (in the
         case of the Company).

                  (c) This Agreement shall be governed by and construed in
accordance with the laws of the State of Delaware.

                  (d) This Agreement constitutes the entire agreement of the
parties with respect to the subject matter hereof


                                       9
<PAGE>   10
and may not be modified or amended except in writing signed by the Company and
by the holders of not less than a majority of the Registerable Stock.

                  (e) This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original, but all of which
together shall constitute one and the same instrument.



                                       10
<PAGE>   11
         Please indicate your acceptance of the foregoing by signing and
returning the enclosed counterpart of this letter, whereupon this letter (herein
sometimes called "this Agreement") shall be a binding agreement between the
Company and you.

                                                   Very truly yours,

                                                   THE BISYS GROUP, INC.


                                               By:/s/ Lynn Magnum
                                                  --------------------
                                                  Name:  Lynn Magnum
                                                  Title: Chairman & CEO

AGREED TO AND ACCEPTED
as of the date first
above written:

                                                  /s/ William J. Nutt
                                                  --------------------
                                                  William J. Nutt



                                                  /s/ Marie E. Connolly
                                                  ----------------------
                                                  Marie E. Connolly



                                                  /s/ John W. Gomez
                                                  -----------------
                                                  John W. Gomez



                                                  /s/ Raymond Grady
                                                  ------------------
                                                  Raymond Grady



                                                  /s/ John Goc
                                                  --------------
                                                  John Goc



                                                  /s/ Frederick Dey
                                                  ----------------
                                                  Frederick Dey





                                       11
<PAGE>   12
                                                  /s/ Donald Thurman
                                                  ------------------
                                                  Donald Thurman



                                                  /s/ Joseph Tower
                                                  -----------------
                                                  Joseph Tower








                                       12
<PAGE>   13
                                   SCHEDULE I

Name and Address of Persons                        Number of Shares of BISYS
---------------------------                        -------------------------
Whom Shares are to be issued                       Common Stock to be Issued
----------------------------                       -------------------------

William J. Nutt                                                  347,824
One Parker Lane
Marblehead, MA 01945

Marie E. Connolly                                                 28,985
6 Williams Road
Wayland, MA 01778

John W. Gomez                                                     57,970
7 Paul Revere Road
Hingham, MA 02043

Raymond Grady                                                      2,028
7 Briarwood Drive
Barrington, RI 02806

John Goc                                                           1,449
20 Forest Street
Saugus, MA 01906

Theodore Dey                                                         289
328 Commerical Street
Boston, MA 02109

Donald Thurman                                                     1,739
9569 Colorow Court
Parker, CO 80134

Joseph Tower                                                       2,318
26 Stone Cleave Road
N. Andover, MA 01845

           Total                                                 442,602