A method of handling Invite messages for a multi-user communication session utilizing the IP Multimedia Subsystem to set up and control the session. Two or more access servers control user access. A first access server receives from a session-initiating user, an Invite that identifies as a potential participant, at least one user group which is owned by a second access server. The first access server sends to the second access server, an Invite that identifies the user group. The second server resolves the group identification into a set of group member identities and sends the identities in a response to the first access server. The first access server then sends Invites to at least some of the group members identified in the response.