A billing center for a mobile communications system is provided which allows groups of subscribers sharing a single account to be identified both during the call rating procedure, during which call detail records are related to add a charge to the call detail record, and/or to add a usage of a pre-paid usage allocation to be added to the call detail records, and during the production of billing details at the end of a billing period. A relational database holds usage allocation records which pertain either to a single subscriber, or to a group of subscribers. The group of subscribers may consist of any of 2 to 50, or more, subscribers, depending on the subscription type held by the group.