Group communication in large-scale interactive applications such as real-time conferencing and collaboration, networked virtual environments (e.g. massively multi-player games) requires efficient, low-overhead group communication mechanisms. In this paper we consider the application-level (or end-system) multicast and propose a stateless group communication mechanism together with its tree building algorithms. Stateless multicast reduces the control signaling of dynamic multicast groups linearly with the group size. To support interactive applications involving a large number of dynamic multicast groups, the application level multicast uses stateless forwarding within clusters of network nodes and hierarchical aggregation of multicast group membership. We show that dynamic tree construction achieve low computation overhead with a controlled degradation of the end-to-end data path performance.