There exists a formal description format for multicast sessions, the Session Description Protocol (SDP, RFC 2327). The most widely adopted manifestation of this protocol is the graphical SDR tool (Session Directory Tool). A user can join an advertisement by clicking the particular entry.
Alternative approaches use web portals or Java applets to display session announcements. For detailed information on these tools and multicast applications in general, visit the excellent repository of the UCL Networked Multimedia Research Group at http://www-mice.cs.ucl.ac.uk/multimedia/software.