It's also related to RFC 1324 . Chatroom servers are pretty simple buggers. In a traditional chat server, these are all put together in a single big server program. IRC expands this so there are several connected servers that act as one big server. I believe this is the wrong paradigm.
Some different approaches follow.
This is the approach that mailing lists generally use. An alternate approach is to use multicast IP, the way the Network Research Group at Lawrence Berkeley Labs did with their wb application, based on their SRM, or Scalable Reliable Multicast, protocol. A third approach is to have each user maintain a list of the members of the group, and send a copy of each message to each group member when they want to say something. The mechanisms for sending messages to groups are generally the same as the mechanisms for sending messages to users, with some sort of pseudo-user representing a group. Except when they're not.