While the advantages of multicast delivery over multiple unicast deliveries is undeniable, the deployment of the IP multicast protocol has been limited to "islands" of network domains under single administrative control. Deployment of inter-domain multicast delivery has been slow due to both technical and administrative reasons. In this paper we propose a Host Multicast Tree Protocol (HMTP) that (1) automates the interconnection of IP-multicast enabled islands and (2) provides multicast delivery to end hosts where IP multicast is not available. With HMTP, end-hosts and proxy gateways of IP multicast-enabled islands can dynamically create shared multicast trees across different islands. Members of an HMTP multicast group self-organize into an efficient, scalable and robust multicast tree. The tree structure is adjusted periodically to accommodate changes in group membership and network topology. Simulation results show that the multicast tree has low cost, and data delivered over it experiences moderately low latency.