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. You can change your nickname with M-x jabber-muc-nick. See Automation, for setting default nicknames.
When trying to join a room, jabber.el first sends a service discovery
info request to the room, to find out whether it exists and what
features are enabled (in particular whether the room is
password-protected). However, this can cause problems with some buggy
MUC services (or services that respond in a way that jabber.el doesn't
expect). A workaround for that is to set
jabber-muc-disable-disco-check to t; however, the bug should be
unearthed and fixed.
Groupchat messages will be displayed in a buffer called
*-jabber-groupchat-:-groupchat-*. It works much like the
chat buffer.
To change the topic of a groupchat, type M-x jabber-muc-set-topic. The current topic is shown in the header line.
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. See MUC Administration, for the meaning of roles and affiliations.