In iChat 4 and higher (Mac OS X 10.5 and 10.6), favorite chat rooms can be saved in the File > Go To Chat Room... window by clicking the "+" sign in the lower left corner. There is also the option to Autojoin a saved room at login by checking the Autojoin box to the right of the saved room.
If you have a Mac OS X Server it's very convenient and easy to setup the iChat (Jabber) service to host a company-wide group chat room that everyone auto-joins when they login to iChat.
If a group chat is too distracting, go to the iChat preferences, under Messages and check the "Watch for my name in incoming messages" box. This will alert you when anyone chats your name (and it's polite that they do use your name when chatting you).