To efficiently support large-scale multicast applications with many thousand simultaneous members, it is essential that protocol mechanisms be available which support efficient exchange of control information between the members of a multicast group. Recently, we proposed the use of a control topology, which organizes multicast group members in a logical n-dimensional hypercube, and transmits all control information along the edges of the hypercube. In this paper, we present the design, verification, and implementation of a protocol, called HyperCast, which maintains members of a large multicast group in a logical hypercube. We use measurement experiments of an implementation of the protocol on a computer cluster of workstations to quantitatively assess the performance of the protocol for multicast group sizes up to 1024 members.