Distributed multi-user interactive systems have a rich and complex set ofrequirements. A promising approach to tackle the complexity of these systems is to rely on configurable architectures that are able to support component re-utilization and composition. The MOOSCo project, Multi-user Object-Oriented environments with Separation of Concerns, addresses the difficulties in applying a component-based approach in a vertical and integrated manner, from analysis to implementation, to the design ofthis class ofsystems. To support communication among distributed components, the project uses a configurable group communication system called Appia. The paper discusses the role of Appia in the MOOSCo architecture and shows how it makes possible to derive, in a simple and elegant way, the most appropriate protocol composition depending on the objects shared by the multi-user object-oriented environment.