In a shared communication service plan in which multiple subscribers operate under a common billing account, a method and system is provided for more equitably allocating service costs among the subscribers in the group, by allocating overage costs based on the extent to which each subscriber used base services available to the group. For instance, if the group plan includes a designated number of base minutes of service per month and defines a per-minute overage cost for minutes used beyond the base number, the method and system may allocate to a given subscriber a portion of the group's overage cost, with the portion being determined based on the portion of the group's base minutes used by the subscriber. Advantageously, an invoice may then be generated setting forth the equitable allocation among the subscribers in the group, thus allowing the group to more readily determine how much each subscriber should pay.