This paper presents a general architectural model for group communications which are communications that may involve more than two parties. The concept of group is presented and the concept of group association, which is an instance of group communication, is analysed. Those group associations are modeled as a set of basic components called multicast conversations. At the service boundery, new architectural concepts are introduced to identify a group association as well as its multicast conversations. Then, facilities to handle the group associations and the multicast conversations are defined and the properties of data transfer on a group association are examined. Finally, the paper deals with aspects of group management.