<PAGE>   1
                                                                   EXHIBIT 10.31


                               ASSIGNMENT OF LEASE

       THIS ASSIGNMENT OF LEASE (this "Assignment") is made effective as of July
1, 1999 (the "Effective Date"), by and between THE ADVISORY BOARD COMPANY
("Assignor") and DGB ENTERPRISES, INC. ("Assignee").

                                    RECITALS

       A.     This Assignment pertains to a certain Office Lease Agreement
between John Hancock Mutual Life Insurance Company ("John Hancock"), as
landlord, and Assignor, as tenant, dated as of August 7, 1992 (as amended, the
"Lease"), pursuant to which Assignor leased certain premises located at 600 New
Hampshire Avenue, N.W., Washington, DC, as more particularly described in the
Lease.

       B.     Assignor, as of the Effective Date, desires to assign all of its
right, title and interest in the Lease to Assignee and Assignee desires to
accept such assignment.

       NOW, THEREFORE, in consideration of the foregoing premises, the mutual
covenants set forth herein, and other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Assignor and Assignee,
agreeing to be legally bound, hereby agree as follows:

              1.     Assignment of Lease.  As of the Effective Date, Assignor
hereby transfers and assigns to Assignee all of its right, title, and interest
in and to the Lease.

              2.     Assumption of Lease. Assignee hereby accepts the foregoing
assignment and agrees, for the benefit of Assignor and John Hancock, to assume
and be responsible for the payment of the rent and other charges payable by
Assignor as tenant under the Lease, and to keep, perform and observe all of the
other terms, covenants, provisions and conditions to be kept, performed or
observed by Assignor as tenant under the Lease. Assignee represents that it has
a true and correct copy of the Lease and all of is parts and represents that it
has read, understands, accepts and agrees to each and every term of the Lease
and all of its parts, without qualification.

              3.     Applicable Law. This Assignment shall be governed by the
applicable laws of the District of Columbia.

              4.     Successors and Assigns.  This Assignment shall be binding
upon and inure to the benefit of the parties hereto and their respective
permitted heirs, successors, legal representatives and assigns.

              5.     Integrated Agreement.  This Assignment contains the entire
integrated agreement between the parties respecting the subject matter of this
Assignment and supersedes all prior understandings and agreements, whether oral
or in writing, between the parties respecting the subject matter of this
Assignment.



<PAGE>   2



       IN WITNESS WHEREOF, Assignor and Assignee have executed this Assignment
under seal effective as of the date first written above.

                                    ASSIGNOR:

                                    THE ADVISORY BOARD COMPANY

                                    By: /s/ JEFFREY D. ZIENTS            (SEAL)
                                       ---------------------------------
                                       Name:  Jeffrey D. Zients
                                       Title:  Chief Executive Officer

                                    ASSIGNEE:

                                    DGB ENTERPRISES, INC.


                                    By: /s/ DAVID BRADLEY                (SEAL)
                                       ---------------------------------
                                       Name:  David Bradley
                                       Title:  Chairman



                                       2