EXHIBIT A

                                                                   EXHIBIT 99(A)

                                                                   Page 18 of 24

                                    EXHIBIT A

AGREEMENT OF REPORTING PERSONS

     Each of the undersigned hereby agrees that Amendment No. 1 to Schedule 13G
filed on the date hereof with respect to the shares of Common Stock of Cogent
Communications Group, Inc. has been filed on behalf of the undersigned.

SIGNATURE:

     Dated: February 7, 2003

Entities:

Oak Investment Partners IX, Limited Partnership Oak Associates IX, LLC Oak IX
Affiliates Fund - A, Limited Partnership Oak IX Affiliates Fund, Limited
Partnership Oak IX Affiliates, LLC Oak Management Corporation

                                        By:     /s/ Xxxxxx X. Xxxxxxxxxx
                                            ---------------------------------
                                                 Xxxxxx X. Xxxxxxxxxx, as
                                                 General Partner or
                                                 Managing Member or as
                                                 Attorney-in-fact for the
                                                 above-listed entities

Individuals:

Bandel X. Xxxxxx
Xxxxxx X. Xxxxxxxxx
Xxxxxx X. Xxxxxxxxxx
Xxxxxxx X. Xxxxxx
Xxx X. Xxxxxx

                                        By:     /s/ Xxxxxx X. Xxxxxxxxxx
                                            ---------------------------------
                                                 Xxxxxx X. Xxxxxxxxxx,
                                                 Individually and as
                                                 Attorney-in-fact for the
                                                 above-listed individuals