This paper presents a formal design for a novel group multicast service that provides virtually synchronous semantics in asynchronous fault-prone environments. The design employs a client-server architecture in which group membership is maintained not by every process but while virtually synchronous group only dedicated membership servers implement multicast implemented by service end-points running at the clients.Specifically, the paper defines service semantics for the client-server interface, that is, for the group membership service. The paper then specifies virtually synchronous semantics for the new group multicast service, as a collection of commonly used safety and liveness properties. Finally, the paper presents new algorithms that use the defined group membership service to implement the specified properties. The algorithm that provides the complete virtually synchronous semantics executes in a single message round in parallel with the membership service's agreement on views, and is therefore more efficient than previously suggested algorithms providing such semantics.