You can select a default nickname by setting jabber-nickname.
Additionally, you can set different nicknames for different groups, by
customizing jabber-muc-default-nicknames. There you specify
the JID of the group, and your preferred nickname.
If you want to automatically join certain rooms when connecting, you
can set jabber-muc-autojoin to a list containing the JIDs of
the rooms you want to enter. To disable this feature, remove
jabber-muc-autojoin from jabber-post-connect-hook.