To illustrate the protocols we will use our friends Alice and Bob to describe the good users and Trent acts as a trusted server. Unfortunately in the real world we also have evil and malicious personalities, represented by Mallory who plays the malicious attacker and Eve who likes to eavesdrop. These personalities are also described in [Sch96].
The terminology used to describe multicast participants is senders or receivers/subscribers if we want to distinguish between the capability of sending vs receiving and we just use member if the distinction is not important in the context. A user may join or subscribe to a multicast group. He can either leave the group voluntarily or get expelled.
The notation for cryptographic methods are the following. When we encrypt a message M with key K we write {M}K. The hash of message M is denoted as H(M).