This paper examines the problem of building scalable, fault-tolerant distributed systems from collections of communicating process groups, while maintaining well-defined end-to-end delivery semantics. We propose a new architecture which supports modular group composition by providing a distinction between intra-group and inter-group communication. With this architecture, multiple group communication protocols and end-to-end delivery semantics can be used in a single system. These features reduce the complexity of ordering messages in a group composition, resulting in enhanced scalability. Finally, we present simulation results comparing the performance of a group composition using our architecture to that of a single process group.