An exemplary method implemented by nodes in a telecommunication network includes associating a plurality of public user identities (PUIDs) into groups where each PUID represents a user served by a group of related endpoints, such as a private branch exchange (PBX). The groups are stored at the first node. Each group contains a group address that uniquely identifies the group and the PBX associated the users with PUIDs that are associated with the group. A first registration request is transmitted from the first node to an internet protocol multimedia subsystem (IMS) where the first registration request includes the group address and the PUIDs that are associated with the group. The first registration request seeks to establish implicit registration in the IMS of the PUIDs associated with the group with the group address. A reply from the IMS is received at the first node in response to the first registration request where the reply seeks authentication for the requested implicit registration of the PUIDs. In response to receipt of the reply, authentication information is calculated and transmitted as a second registration request that contains the authentication information and the group address and the PUIDs that are associated with the group. The PUIDs in association with the group address are implicitly registered in the IMS upon a determination that the authentication information is acceptable.