A system and method for assigning a unique identifier to a prospective user of an instant messaging system. The IM system comprises a plurality of clients (111) having IM applications of the same or different types, selectively interconnected to an IM server (113) by way of the internet (117). The system includes a registration handler (119) for receiving a client specific access address (johnsmith@company.com) of a prospective user (127) on the internet (117) together with a request (115) to register or tentatively register the prospective user (127). A unique identifier assigner is provided to automatically allocate a unique numeric identifier (UIN) to the prospective user (127) and match the UIN to the client specific address (johnsmith@company.com) of the prospective user. An account processer registers or tentatively registers an account for the prospective user (127) in a user database (123) for future use (125) by storing (121) the matched UIN and client specific access address under the UIN in the database. A message despatcher sends a notification (129) of the UIN to the prospective user (127) at the client specific address of the prospective user, either: confirming the registering of the prospective user if the initial sending was associated with a direct request to register from the prospective user; or inviting registration of the prospective user if the initial sending was associated with a request to register from someone other than the prospective user.