You can configure jabber.el to use a certain nickname for a certain room, or to automatically join a certain room when you connect. You can do this either by storing bookmarks on the server or by setting Emacs variables.
Type M-x jabber-edit-bookmarks to add bookmarks. You can specify the JID of the conference, the name of the conference (not used by jabber.el), whether to automatically join the room, your desired nickname (or leave empty), and the room password (or leave empty).
The default nickname for groupchats is the username part of your JID.
If you don't use bookmarks, you can set different nicknames for
different groups by customizing
There you specify the JID of the group, and your preferred nickname.
Automatically joining certain rooms when connecting can be accomplished
jabber-muc-autojoin to a list containing the JIDs of
the rooms you want to enter. To disable this feature, remove
Please note, that
jabber-muc-autojoin are machine-local, but apply to all
accounts—if you connect several accounts, both will try to connect to
the same chat rooms, or use the same nickname. This will lead to