In XMPP, there is not a way that I know of to force people to join a chat room. You can, however, strongly suggest that they join a chat room, by giving them a bookmark that has the ‘autojoin’ feature enabled. Most clients will follow those instructions.
Bookmarks are typically managed by individual users. You can also use the Openfire Bookmarks plugin to, in the server, define bookmarks for users of your server.