Generally speaking there are two ways to set up a session, although hybrids of the two are possible: announce the session to all possible participants, or invite just those participants you wish to be in the session.
To announce a session to all possible participants, the Session Announcement Protocol is used. This sends out announcement information to anyone who is listening to the right multicast group giving them information about the session.
To invite someone into a session, the Session Initiation Protocol is used. The sends an invitation to participate in a session to a specific user.
Both of these protocols carry the description of the session in a precise format called the Session Description Protocol. This isn't really a protocol, but a well-defined format for describing which media are in a session, information about the protocols and formats the session will use, and enough information to decide whether or not to participate in the session.