The groupchat menu can be accessed by typing C-c C-m in the roster buffer. You can also type the commands directly, as will be shown here.
To join a groupchat, type M-x jabber-groupchat-join. You will be prompted for the groupchat to join, and your nickname in the groupchat. This nickname doesn't need to have any correlation to your JID; in fact, groupchats are usually (but not always) configured such that only moderators can see your JID.
Groupchat messages will be displayed in a buffer called
*-jabber-groupchat-:-groupchat-*. It works much like the
chat buffer.
To leave a groupchat, type M-x jabber-groupchat-leave.
If you are the owner of a groupchat, you can change its configuration by typing M-x jabber-groupchat-get-config. A configuration form will be rendered in new buffer.
To see which people are in a groupchat, type M-x jabber-muc-names. This gives a list of nicknames, “roles”, “affiliations”, and possibly JIDs. The possible roles are “moderator”, “participant” (has voice), “visitor” (doesn't have voice), and “none” (not in room). Affiliations are “owner”, “admin”, “member”, “outcast” (banned), and “none”.
If you have moderator privileges, you can change the role of a participant with M-x jabber-muc-set-role. Kicking means setting the role to “none”. Granting and revoking voice are “participant” and “visitor”, respectively. Changing affiliations is not yet supported.