The notion of Group communication has long been introduced as a core service of distributed systems. More recently, this notion appeared with a somewhat different meaning in the field of mobile ad hoc systems. In this context, we study the group membership problem. After specifying the basic safety properties of such groups and a maximality criterion based on cliques, we propose a group membership algorithm. Lastly, with respect to this criterion, we compare our algorithm with two group membership algorithms for ad hoc environments. Moreover, a formal description in TLA+ has been programmed and verified by model-checking for small networks.