A communication system including a first administrative subsystem and a second administrative subsystem providing group communication service. The system includes a first server for storing group and list management information of subscribers of the first administrative subsystem. The system also includes a second server for storing group and list management information of subscribers of the second administrative subsystem. Further, the system includes an interface for exchanging group and list management information between the first server and the second server. In addition, the system reduces ambiguity in group information management of integrated systems that include two or more administrative domains.