You can invite someone to a groupchat with M-x jabber-muc-invite (also available in the MUC menu). Pay attention to the order of the arguments—as both users and rooms are just JIDs, it is technically possible to invite a room to a user, but that's probably not what you want.
When you receive an invitation, it appears in the chat buffer along with two buttons, “Accept” and “Decline”. Pressing “Accept” enters the room, as you would expect. Pressing “Decline” gives you an opportunity to state the reason why you're not joining.