A group including a larger number of processes implies larger computation and communication overheads to manipulate and transmit messages. We discuss a group which is composed of subgroups of processes to reduce the overheads. Each subgroup has a gateway process which communicates with the other gateway processes. We propose a protocol to causally deliver messages to processes in a group by using a vector of message sequence numbers whose size is the number of subgroups, smaller than number of processes. We evaluate the protocol.