For a user to join the group check out
What method, through to a jid, access can query to join the group?
Thank you very much
Added by Wojciech Kapcia about 3 years ago
If you are asking if you can force user being in the room externally then it's not possible now.
send invitation to the particular JID;
you could utilize HTTP API - send any stanza feature to send room-join presence on behalf of the user but you would need to know user's full JID before.