<PAGE>

                                                                   EXHIBIT 10.31

                                                                  Execution Copy
                                                                  --------------

                            STOCKHOLDERS AGREEMENT
                            ----------------------


          THIS STOCKHOLDERS AGREEMENT (this "Agreement") is made as of March 23,
                                             ---------
1999 by and among (i) ZC Acquisition Corp., a Delaware corporation (the
"Company"), (ii) GTCR Fund VI, L.P., a Delaware limited partnership ("GTCR"),
 -------                                                              ----
GTCR VI Executive Fund, L.P., a Delaware limited partnership ("Executive Fund")
                                                               --------------
and GTCR Associates VI, a Delaware general partnership ("Associates Fund") (each
                                                         ---------------
an "Investor" and collectively, the "Investors"),  (iii) William Seibel and any
    --------                         ---------
other executive employee of the Company who, at any time, acquires securities of
the Company in accordance with Section 9 hereof and executes a counterpart of
                               ---------
this Agreement or otherwise agrees to be bound by this Agreement (each, an
"Executive" and collectively, the "Executives"), and (iv) each of the other
 ---------                         ----------
entities and individuals set forth from time to time on the attached "Schedule
                                                                      --------
of Stockholders" under the heading "Other Stockholders" who, at any time,
---------------
acquires securities of the Company in accordance with Section 9 hereof and
                                                      ---------
executes a counterpart of this Agreement or otherwise agrees to be bound by this
Agreement.  The Investors, the Executives and the other entities and individuals
listed on the Schedule of Stockholders are collectively referred to herein as
the "Stockholders" and individually as a "Stockholder."  Capitalized terms used
     ------------                         -----------
but not otherwise defined herein are defined in Section 7 hereof.
                                                ---------

          The Investors will purchase shares of the Company's Common Stock, par
value $.01 per share (the "Common Stock"), and the Company's Class A Preferred
                           ------------
Stock (the "Class A Preferred"), pursuant to a Purchase Agreement between the
            -----------------
Investors and the Company dated as of the date hereof (the "Purchase
                                                            --------
Agreement").  Each Executive will purchase shares of Common Stock pursuant to an
---------
agreement between the Company and such Executive (each an "Executive Purchase
                                                           ------------------
Agreement" and collectively the "Executive Purchase Agreements").
---------                        -----------------------------

          The execution and delivery of this Agreement is a condition to the
Investors' purchase of Common Stock and Class A Preferred pursuant to the
Purchase Agreement.

          NOW, THEREFORE, in consideration of the mutual covenants contained
herein and other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties to this Agreement hereby agree as
follows:

          1.   Board of Directors.
               ------------------

          (a)  From and after the Closing and until the provisions of this
Section 1 cease to be effective, each Stockholder shall vote all of his
---------
Stockholder Shares and any other voting securities of the Company over which
such Stockholder has voting control and shall take all other necessary or
desirable actions within his control (whether in his capacity as a stockholder,
director, member of a board committee or officer of the Company or otherwise,
and including, without limitation, attendance at meetings in person or by proxy
for purposes of obtaining a quorum and execution of written consents in lieu of
meetings), and the Company shall take all necessary and desirable actions within
its control (including, without limitation, calling special board and
stockholder meetings), so that:
<PAGE>

               (i)   the authorized number of directors on the Company's board
     of directors (the "Board") shall be established at seven directors;
                        -----

               (ii)  the following persons shall be elected to the Board:

                     (A) two representatives designated by GTCR (the "Investor
                                                                      --------
          Directors"), who shall initially be Philip A. Canfield and Timothy P.
          ---------
          McAdam;

                     (B) William Seibel and one additional Executive of the
          Company designated by the Company's chief executive officer (the
          "CEO") (each, an "Executive Director" and collectively, the "Executive
           ---              ------------------                         ---------
          Directors"); and
          ---------

                     (C) three representatives chosen jointly by GTCR and the
               CEO (the "Outside Directors"); provided that no Outside Directors
                         -----------------    --------
               shall be a member of the Company's management or an employee or
               officer of the Company or its subsidiaries; provided further that
                                                           --------
               if GTCR and the CEO are unable to agree on the Outside Directors
               within 10 days after the date specified by GTCR for electing the
               Outside Directors, then GTCR, in its sole discretion, shall
               designate the Outside Directors;

               (iii) the composition of any committee of the Board shall include
     at least two Investor Directors;

               (iv)  the composition of the board of directors of each of the
     Company's subsidiaries (a "Sub Board") shall be the same as that of the
                                ---------
     Board;

               (v)   the removal from the Board, a Sub Board or a committee
     (with or without cause) of any Investor Director or any Outside Director
     shall be upon (and only upon) the written request of GTCR;

               (vi)  if any Executive Director ceases to hold his respective
     executive office, he shall be removed promptly after such time from the
     Board, each Sub Board and each committee; and

               (vii) in the event that any representative designated hereunder
     for any reason ceases to serve as a member of the Board, a Sub Board or a
     committee during his term of office, the resulting vacancy on the Board,
     the Sub Board or such committee shall be filled by a representative
     designated by the person or persons originally entitled to designate such
     director pursuant to Section 1(a)(ii) above.
                          ----------------

          (b)  The Company shall pay all out-of-pocket expenses incurred by each
director in connection with attending regular and special meetings of the Board,
any Sub Board and any committee thereof.

                                      -2-
<PAGE>

          (c)  If any party fails to designate a representative to fill a
directorship pursuant to the terms of this Section 1, the election of a person
                                           ---------
to such directorship shall be accomplished in accordance with the Company's
bylaws and applicable law.

          (d)  The provisions of this Section 1 shall terminate upon first to
                                      ---------
occur of (i) the consummation of a Public Offering or (ii) the consummation of a
Sale of the Company.

          2.   Legend.  Each certificate evidencing Stockholder Shares and each
               ------
certificate issued in exchange for or upon the transfer of any Stockholder
Shares (if such shares remain Stock  holder Shares as defined herein after such
transfer) shall be stamped or otherwise imprinted with a legend in substantially
the following form:

          "The securities represented by this certificate are subject
          to a Stockholders Agreement dated as of [date of issuance]
          among the issuer of such securities (the "Company") and certain
                                                    -------
          of the Company's stockholders.  A copy of such Stockholders
          Agreement will be furnished without charge by the Company to
          the holder hereof upon written request."

The Company shall imprint such legend on certificates evidencing Stockholder
Shares outstanding prior to the date hereof.  The legend set forth above shall
be removed from the certificates evidencing any shares which cease to be
Stockholder Shares.

          3.   Participation Rights.
               --------------------

          (a)  At least 30 days prior to any Transfer of Stockholder Shares by
GTCR, GTCR shall deliver a written notice (the "Sale Notice") to the Company and
                                                -----------
the other Stockholders (the "Other Stockholders") specifying in reasonable
                             ------------------
detail the identity of the prospective transferee(s) and the terms and
conditions of the Transfer.  The Other Stockholders may elect to participate in
the contemplated Transfer by delivering written notice to GTCR within 30 days
after delivery of the Sale Notice.  If any Other Stockholders have elected to
participate in such Transfer, GTCR and such Other Stockholders will be entitled
to sell in the contemplated Transfer, at the same price and on the same terms, a
number of shares of Common Stock equal to the product of (A) the quotient
determined by dividing the number of shares of Stockholders Shares owned by such
person by the aggregate number of outstanding shares of Stockholder Shares owned
by GTCR and the Other Stockholders participating in such sale and (B) the number
of Stockholder Shares to be sold in the contemplated Transfer.

          (b)  GTCR will use reasonable best efforts to obtain the agreement of
the prospective transferee(s) to the participation of the Other Stockholders in
any contemplated Transfer, and GTCR will not transfer any of its Stockholder
Shares to the prospective transferee(s) unless (A) the prospective transferee(s)
agrees to allow the participation of the Other Stockholders or (B) GTCR agrees
to purchase the number of such class of Stockholder Shares from the Other
Stockholders which the Other Stockholders would have been entitled to sell
pursuant to this Section 3(b) for the consideration per share to be paid to GTCR
                 ------------
by the prospective transferee(s).

                                      -3-
<PAGE>

          (c)  Notwithstanding anything to the contrary in any other provision
of this Agreement, the restrictions set forth in this Section 3 shall not apply
                                                      ---------
to (i) any Transfer of Stockholder Shares by GTCR to or among its Affiliates or
(ii) a Public Sale; provided that the restrictions contained in this Agreement
                    --------
will continue to be applicable to the Stockholder Shares after any Transfer
pursuant to clause (i) and the transferee of such Stockholder Shares shall agree
in writing to be bound by the provisions of this Agreement.  Upon the Transfer
of Stockholder Shares pursuant to clause (i) of the previous sentence, the
transferees will deliver a written notice to the Company, which notice will
disclose in reasonable detail the identity of such transferee.

          (d)  The provisions of this Section 3 will terminate upon the first to
                                      ---------
occur of (i) the consummation of a Sale of the Company and (ii) the consummation
of a Public Offering.

          4.   First Refusal Rights.
               --------------------

          (a)  Prior to making any Transfer of Stockholder Shares (other than a
Transfer pursuant to a Public Sale of the type referred to in clause (i) of the
definition thereof or a Sale of the Company), any Stockholder (other than the
Investors) desiring to make such Transfer (the "Transferring Stockholder") will
                                                ------------------------
give written notice (the "Sale Notice") to the Company, the Executives and the
                          -----------
Investors (each a "Sale Notice Recipient", and collectively, the "Sale Notice
                   ---------------------                          -----------
Recipients").  The Sale Notice will disclose in reasonable detail the identity
----------
of the prospective transferee(s), the number of shares to be transferred and the
terms and conditions of the proposed transfer.  Such Transferring Stockholder
will not consummate any Transfer until 45 days after the Sale Notice has been
given to the Sale Notice Recipients, unless the parties to the Transfer have
been finally determined pursuant to this Section 4 prior to the expiration of
                                         ---------
such 45-day period.  (The date of the first to occur of such events is referred
to herein as the "Authorization Date").
                  ------------------

          (b)  The Company may elect to purchase all (but not less than all) of
such Stockholder Shares to be transferred upon the same terms and conditions as
those set forth in the Sale Notice by delivering a written notice of such
election to the Executives and the Investors (individually, an "Other Sale
                                                                ----------
Notice Recipient", and collectively, the "Other Sale Notice Recipients") and to
----------------                          ----------------------------
the Transferring Stockholder within 20 days after the Sale Notice has been given
to the Company.  If the Company has not elected to purchase all of the
Stockholder Shares to be transferred, the Other Sale Notice Recipients may elect
to purchase all (but not less than all) of the Stockholder Shares to be
transferred upon the same terms and conditions as those set forth in the Sale
Notice by giving written notice of such election to such Transferring
Stockholder within 25 days after the Sale Notice has been given to GTCR.  If
more than one Other Sale Notice Recipient elects to purchase the Stockholder
Shares to be transferred, the shares of Stockholder Shares to be sold shall be
allocated among the Other Sale Notice Recipients pro rata according to the
number of shares of Common Stock owned by each Other Sale Notice Recipient on a
fully diluted basis.  If neither the Company nor the Other Sale Notice
Recipients elects to purchase all of the Stockholder Shares specified in the
Sale Notice, the Transferring Stockholder may transfer the Stockholder Shares
specified in the Sale Notice at a price and on terms no more favorable to the
transferee(s) thereof than specified in the Sale Notice during the 60-day period
immediately following the Authorization Date.  Any Stockholder Shares not
transferred within such 60-day period will be subject to the provisions of this
Section 4 upon subsequent transfer.  The Company may pay the purchase price for
---------

                                      -4-
<PAGE>

such shares by offsetting amounts outstanding under any bona fide debts owed by
the Transferring Stockholder to the Company.

          (c)  The restrictions of this Section 4 will not apply with respect to
                                        ---------
(i) any Transfer of Stockholder Shares by any Stockholder to or among its
Affiliates, (ii) any Transfer of Stockholder Shares by or to any Investor, (iii)
a repurchase of Stockholder Shares by the Company pursuant to the terms of the
Management Agreements (as defined in the Purchase Agreement), (iv) a Public
Sale, or (v) an Approved Sale (as defined in Section 5(a)); provided that the
                                             -------------  --------
restrictions contained in this Agreement will continue to be applicable to the
Stockholder Shares after any Transfer pursuant to clause (i) or (ii) above and
the transferee of such Stockholder Shares shall agree in writing to be bound by
the provisions of this Agreement.  Upon the Transfer of Stockholder Shares
pursuant to clause (i) or (ii) of the previous sentence, the transferees will
deliver a written notice to the Company, which notice will disclose in
reasonable detail the identity of such transferee.

          (d)  Notwithstanding anything herein to the contrary, except pursuant
to clause (c) above, in no event shall any Transfer of Stockholder Shares
pursuant to this Section 4 be made for any consideration other than cash payable
                 ---------
upon consummation of such Transfer.

          (e)  The restrictions set forth in this Section 4 shall continue with
                                                  ---------
respect to each Stockholder Share until the earlier of (i) the date on which
such Share has been transferred in a Public Sale, (ii) the consummation of an
Approved Sale, and (iii) the date on which such Stockholder Share has been
transferred pursuant to this Section 4 (other than Section 4(c)).
                             ---------             ------------

          5.   Sale of the Company.
               -------------------

          (a)  If the holders of a majority of the shares of Common Stock then
outstanding approve a Sale of the Company (an "Approved Sale"), each holder of
                                               -------------
Common Stock shall vote for, consent to and raise no objections against such
Approved Sale.  If the Approved Sale is structured as a (i) merger or
consolidation, each holder of Common Stock shall waive any dissenters' rights,
appraisal rights or similar rights in connection with such merger or
consolidation or (ii) sale of stock, each holder of Common Stock shall agree to
sell all of his Common Stock and rights to acquire Common Stock on the terms and
conditions approved by the Board and the holders of a majority of the Common
Stock (voting as a single class) then outstanding. Each holder of Common Stock
shall take all necessary or desirable actions in connection with the
consummation of the Approved Sale as requested by the Company.

          (b)  The obligations of the holders of Common Stock with respect to
the Approved Sale of the Company are subject to the satisfaction of the
following conditions: (i) upon the consummation of the Approved Sale, each
holder of Common Stock shall receive the same form of consideration and the same
amount of consideration per share, and each holder of Class A Preferred shall
receive consideration not in excess of the aggregate Liquidation Value (as
defined in the Company's Certificate of Incorporation as of the date hereof) of
all shares of Class A Preferred held by such holder (plus all accrued and unpaid
dividends thereon); (ii) if any holders of a class of Common Stock are given an
option as to the form and amount of consideration to be received, each holder of
such class of Common Stock shall be given the same option; and (iii) each holder
of then

                                      -5-
<PAGE>

currently exercisable rights to acquire shares of a class of Common Stock
shall be given an opportunity to either (A) exercise such rights prior to the
consummation of the Approved Sale and participate in such sale as holders of
such class of Common Stock or (B) upon the consummation of the Approved Sale,
receive in exchange for such rights consideration equal to the amount determined
by multiplying (1) the same amount of consideration per share of a class of
Common Stock received by holders of such class of Common Stock in connection
with the Approved Sale less the exercise price per share of such class of Common
Stock of such rights to acquire such class of Common Stock by (2) the number of
shares of such class of Common Stock represented by such rights.

          (c)  If either the Company or the holders of the Common Stock or the
Class A Preferred enter into an negotiation or transaction for which Rule 506
(or any similar rule then in effect) promulgated by the Securities and Exchange
Commission may be available with respect to such negotiation or transaction
(including a merger, consolidation or other reorganization), the holders of
Stockholder Shares will, at the request of the Company, appoint a purchaser
representative (as such term is defined in Rule 501) reasonably acceptable to
the Company.  If any holder of Stockholder Shares appoints a purchaser
representative designated by the Company, the Company will pay the fees of such
purchaser representative, but if any holder of Stockholder Shares declines to
appoint the purchaser representative designated by the Company such holder will
appoint another purchaser representative, and such holder will be responsible
for the fees of the purchaser representative so appointed.

          (d)  Holders of Stockholder Shares will bear their pro rata share
(based upon the number of shares sold) of the costs of any sale of such
Stockholder Shares pursuant to an Approved Sale to the extent such costs are
incurred for the benefit of all holders of Stockholder Shares and are not
otherwise paid by the Company or the acquiring party.  For purposes of this
Section 5(d), costs incurred in exercising reasonable efforts to take all
------------
actions in connection with the consummation of an Approved Sale in accordance
with Section 5(a) shall be deemed to be for the benefit of all holders of the
     ------------
Stockholder Shares.  Costs incurred by holders of Stockholder Shares on their
own behalf will not be considered costs of the transaction hereunder.

          6.   Public Offering.  In the event that the Board or the holders of a
               ---------------
majority of the Investor Registrable Securities (as defined in the Registration
Agreement, dated March 23, 1999, by and among the Company, the Investors and
certain other stockholders) (collectively, the "Approving Persons") approves an
                                                -----------------
initial Public Offering, the holders of Stockholder Shares shall take all
necessary or desirable actions requested by the Approving Persons in connection
with the consummation of such Public Offering, including without limitation
compliance with the requirements of all laws and regulatory bodies which are
applicable or which have jurisdiction over such Public Offering.  In the event
that such Public Offering is an underwritten offering and the managing
underwriters advise the Company in writing that in their opinion the Company's
capital structure would adversely affect the marketability of the offering, each
holder of Stockholder Shares shall consent to and vote for a recapitalization,
reorganization or exchange (each, a "Recapitalization") of any class of the
                                     ----------------
Company's capital stock into securities that the managing underwriters and the
Approving Persons find acceptable and shall take all necessary and desirable
actions in connection with the consummation of such Recapitalization; provided
                                                                      --------
that, each holder
----

                                      -6-
<PAGE>

of Common Stock shall receive the same type of security with the same value per
share, and each holder of Class A Preferred shall receive the same type of
security with the same value per share; provided further, that the aggregate
value of the securities issued to the holders of Class A Preferred shall not
exceed the aggregate Liquidation Value of such Class A Preferred (plus all
accrued and unpaid dividends thereon) or the number of shares of Common Stock
equal to (i) the aggregate Liquidation Value of such Class A Preferred (plus all
accrued and unpaid dividends thereon), divided by (ii) the price per
                                       ------- --
share of the Common Stock to the public in the Public Offering.

          7.   Definitions.
               -----------

          "Affiliate" means, (i) with respect to any Person, means any Person
           ---------
that controls, is controlled by or is under common control with such Person or
an Affiliate of such Person, (ii) with respect to any Investor, means any
general or limited partner of such Investor or any other person, entity or
investment fund controlling, controlled by or under common control with such
Investor, and (iii) with respect to any Executive, means any member of such
Executive's immediate family or a trust in which the beneficiary of such trust
is such Executive or such Executive's immediate family.

          "Closing" shall have the meaning set forth in the Purchase Agreement.
           -------

          "Person" means an individual, a partnership, a limited liability
           ------
company, a corporation, an association, a joint stock company, a trust, a joint
venture, an unincorporated organization and a governmental entity or any
department, agency or political subdivision thereof.

          "Public Offering" means the sale in an underwritten public offering
           ---------------
registered under the Securities Act of shares of the Company's Common Stock
approved by the Board.

          "Public Sale" means any sale of Stockholder Shares (i) to the public
           -----------
pursuant to an offering registered under the Securities Act or (ii) to the
public through a broker, dealer or market maker pursuant to the provisions of
Rule 144 (other than Rule 144(k) prior to a Public Offering) adopted under the
Securities Act.

          "Sale of the Company" means any transaction or series of transactions
           -------------------
pursuant to which any person(s) or entity(ies) (including any Affiliate of any
Investor), other than any Investor, in the aggregate acquire(s) (i) capital
stock of the Company possessing the voting power (other than voting rights
accruing only in the event of a default or breach) to elect a majority of the
Company's board of directors (whether by merger, consolidation, reorganization,
combination, sale or transfer of the Company's capital stock, shareholder or
voting agreement, proxy, power of attorney or otherwise) or (ii) all or
substantially all of the Company's assets determined on a consolidated basis.

          "Securities Act" means the Securities Act of 1933, as amended from
           --------------
time to time.

          "Stockholder Shares" means (i) any Common Stock purchased or otherwise
           ------------------
acquired by any Stockholder, (ii) any equity securities issued or issuable
directly or indirectly with respect to the Common Stock referred to in clause
(i) above by way of stock dividend or stock split or in connection with a
combination of shares, recapitalization, merger, consolidation or other
reorgan-

                                      -7-
<PAGE>

ization, and (iii) any other shares of any class or series of capital stock of
the Company held by a Stockholder; provided that Stockholder Shares shall not
                                   --------
include nonvoting stock described in (iii) above for purposes of Section 1
                                                                 ---------
hereof.  As to any particular shares constituting Stockholder Shares, such
shares will cease to be Stockholder Shares when they have been (x) effectively
registered under the Securities Act and disposed of in accordance with the
registration statement covering them or (y) sold to the public through a broker,
dealer or market maker pursuant to Rule 144 (or any similar provision then in
force) under the Securities Act.

          "Transfer" means to sell, transfer, assign, pledge or otherwise
           --------
dispose of (whether with or without consideration and whether voluntarily or
involuntarily or by operation of law).

          8.   Transfers; Transfers in Violation of Agreement.  Prior to
               ----------------------------------------------
transferring any Stockholder Shares to any person or entity, the Transferring
Stockholder shall cause the prospective transferee to execute and deliver to the
Company and the other Stockholders a counterpart of this Agreement.  Any
transfer or attempted transfer of any Stockholder Shares in violation of any
provision of this Agreement shall be void, and the Company shall not record such
transfer or treat any purported transferee of such Stockholder Shares as the
owner of such shares for any purpose.

          9.   Additional Stockholders.  In connection with the issuance of any
               -----------------------
additional equity securities of the Company, the Company, with the consent of
GTCR, may permit such person to become a party to this Agreement and succeed to
all of the rights and obligations of a "Stockholder" under this Agreement by
obtaining an executed counterpart signature page to this Agreement, and, upon
such execution, such person shall for all purposes be a "Stockholder" party to
this Agreement.

          10.  Amendment and Waiver.  Except as otherwise provided herein, no
               --------------------
modification, amendment or waiver of any provision of this Agreement shall be
effective against the Company or the Stockholders unless such modification,
amendment or waiver is approved in writing by the Company and the holders of a
majority of the Stockholder Shares.  The failure of any party to enforce any of
the provisions of this Agreement shall in no way be construed as a waiver of
such provisions and shall not affect the right of such party thereafter to
enforce each and every provision of this Agreement in accordance with its terms.

          11.  Severability.  Whenever possible, each provision of this
               ------------
Agreement shall be interpreted in such manner as to be effective and valid under
applicable law, but if any provision of this Agreement is held to be invalid,
illegal or unenforceable in any respect under any applicable law or rule in any
jurisdiction, such invalidity, illegality or unenforceability shall not affect
any other provision or any other jurisdiction, but this Agreement shall be
reformed, construed and enforced in such jurisdiction as if such invalid,
illegal or unenforceable provision had never been herein.

          12.  Entire Agreement.  Except as otherwise expressly set forth
               ----------------
herein, this document embodies the complete agreement and understanding among
the parties hereto with respect to the subject matter hereof and supersedes and
preempts any prior understandings, agreements or representations by or among the
parties, written or oral, which may have related to the subject matter hereof in
any way.

                                      -8-
<PAGE>

          13.  Successors and Assigns.  Except as otherwise provided herein,
               ----------------------
this Agreement shall bind and inure to the benefit of and be enforceable by the
Company and its successors and assigns and the Stockholders and any subsequent
holders of Stockholder Shares and the respective successors and assigns of each
of them, so long as they hold Stockholder Shares.

          14.  Counterparts.  This Agreement may be executed in separate
               ------------
counterparts each of which shall be an original and all of which taken together
shall constitute one and the same agreement.

          15.  Remedies.  The Company, each Investor, each Executive and each
               --------
Other Stockholder shall be entitled to enforce their rights under this Agreement
specifically to recover damages by reason of any breach of any provision of this
Agreement and to exercise all other rights existing in their favor.  The parties
hereto agree and acknowledge that money damages may not be an adequate remedy
for any breach of the provisions of this Agreement and that the Company, each
Investor, each Executive and each Other Stockholder may in its sole discretion
apply to any court of law or equity of competent jurisdiction for specific
performance and/or injunctive relief (without posting a bond or other security)
in order to enforce or prevent any violation of the provisions of this
Agreement.

          16.  Notices.  Any notice provided for in this Agreement shall be in
               -------
writing and shall be either personally delivered, or mailed first class mail
(postage prepaid) or sent by reputable overnight courier service (charges
prepaid) to the Company at the address set forth below and to any other
recipient at the address indicated on the schedules hereto and to any subsequent
holder of Common Stock subject to this Agreement at such address as indicated by
the Company's records, or at such address or to the attention of such other
person as the recipient party has specified by prior written notice to the
sending party.  Notices will be deemed to have been given hereunder when
delivered personally, three days after deposit in the U.S. mail and one day
after deposit with a reputable overnight courier service.  The Company's address
is:

          If to the Company:
          -----------------

          ZC Acquisition Corp.
          1048 Tremont Street
          Duxbury, MA 02332
          Attention: General Counsel

                                      -9-
<PAGE>

          with copies to:
          --------------

          GTCR Fund VI, L.P.
          GTCR VI Executive Fund, L.P.
          GTCR Associates VI
          c/o GTCR Golder Rauner, L.L.C.
          6100 Sears Tower
          Chicago, IL  60606-6402
          Attention:  Philip A. Canfield
                      Timothy P. McAdam

          Kirkland & Ellis
          200 East Randolph Drive
          Chicago, IL  60601
          Attention:  Stephen L. Ritchie

          Hale and Dorr LLP
          60 State Street
          Boston, Massachusetts  02109
          Attention:  David E. Redlick

          17.  Governing Law.  The laws of Delaware shall govern all issues
               -------------
concerning the relative rights of the Company and its stockholders and all other
questions concerning the construction, validity and interpretation of this
Agreement, without giving effect to any choice of law or other conflict of law
provision or rule (whether of the State of Delaware or any other jurisdiction)
that would cause the application of the laws of any jurisdiction other than the
State of Delaware.

          18.  Descriptive Headings.  The descriptive headings of this Agreement
               --------------------
are inserted for convenience only and do not constitute a part of this
Agreement.

                               *   *   *   *   *

                                      -10-
<PAGE>

       IN WITNESS WHEREOF, the parties hereto have executed this Stockholders
Agreement on the day and year first above written.


                              ZC ACQUISITION CORP.

                              By:   /s/ William Seibel
                                 -------------------------------

                              Name:     William Seibel
                                   -----------------------------
                              Its:  President

                              GTCR FUND VI, L.P.

                              By:  GTCR Partners VI, L.P.
                              Its: General Partner

                              By:  GTCR Golder Rauner, L.L.C.
                              Its: General Partner

                              By:   /s/ Philip A. Canfield
                                 -------------------------------

                              Name:     Philip A. Canfield
                                   -----------------------------
                              Its:  Principal

                              GTCR VI EXECUTIVE FUND, L.P.

                              By:  GTCR Partners VI, L.P.
                              Its: General Partner

                              By:  GTCR Golder Rauner, L.L.C.
                              Its: General Partner

                              By:   /s/ Philip A. Canfield
                                 -------------------------------

                              Name:     Philip A. Canfield
                                   -----------------------------
                              Its:  Principal

                              GTCR ASSOCIATES VI

                              By:  GTCR Partners VI, L.P.
                              Its: Managing General Partner

                              By:  GTCR Golder Rauner, L.L.C.
                              Its: General Partner

                              By:   /s/ Philip A. Canfield
                                 -------------------------------

                              Name:     Philip A. Canfield
                                   -----------------------------
                              Its:  Principal

                              /s/  William Seibel
                              ----------------------------------
                              William Seibel

                  SIGNATURE PAGE TO THE STOCKHOLDERS AGREEMENT


<PAGE>

                              HEIDRICK & STRUGGLES, INC.

                              By: /s/ Richard D. Nelson
                                 -------------------------------

                              Name:   Richard D. Nelson
                                   -----------------------------

                              Its:    Secretary
                                  ------------------------------


                                CONTINUATION OF
                  SIGNATURE PAGE TO THE STOCKHOLDERS AGREEMENT


<PAGE>

                                                                  EXECUTION COPY
                                                                  --------------

                            SCHEDULE OF STOCKHOLDERS
                            ------------------------


GTCR FUND VI, L.P.
6100 Sears Tower
Chicago, IL  60606-6402
Attention:  Philip A. Canfield
            Timothy P. McAdam

GTCR VI EXECUTIVE FUND, L.P.
6100 Sears Tower
Chicago, IL  60606-6402
Attention:  Philip A. Canfield
            Timothy P. McAdam

GTCR ASSOCIATES VI
6100 Sears Tower
Chicago, IL  60606-6402
Attention:  Philip A. Canfield
            Timothy P. McAdam

William Seibel
7 Amherst Road
Wellesley, MA  02181

OTHER STOCKHOLDERS
------------------

HEIDRICK & STRUGGLES, INC.
233 South Wacker Drive, Suite 4200
Chicago, IL  60606-6303
Attention: Corporate Counsel

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