A group including a larger number of processes implies larger computation and communication overheads O(n2) required to manipulate and transmit messages for a number n of processes. In this paper, we discuss a group which is composed of subgroups of processes. Each subgroup has a gateway process which communicates with the other gateway processes. We propose a mechanism to causally deliver messages to processes in a group by using a vector of message sequence numbers whose size is the number of subgroups, not the number of processes. We assume that networks are less-reliable; i.e., that messages may be lost.