The basic concepts of IP multicast have not radically changed since Steve Deering first suggested them more than 10 years ago.
Receivers use a group membership protocol to inform the network when they wish to join a group.
The network, in turn, runs a multicast routing protocol that is distinct from the unicast routing protocol. The former is used to build and maintain a distribution tree rooted at the source with branches that take the shortest, loop-free paths down to networks where group members exist. Routers perform normal unicast forwarding based on the destination address contained in the packet header. Multicast forwarding is different because it forwards a packet based on both the source and destination addresses in the packet header. The destination group address is used to select which outbound interfaces to forward a copy of the packet to.