This function can be called to process a join invitation once you have successfully called xboxlive_matchmaking_start(). You supply the local UserID and the the invitation values (ID and host ID) which were returned when the invitation was received, along with the name of the Session Template that you created (a string). It will then join the user to the session they've been invited to, and you will receive Social Asynchronous Events as if you had called xboxlive_matchmaking_find().
xboxlive_chat_add_user_to_channel(user_who_received_invite, invitation_id, invitation_host, sda_template_name);
Argument Description user_who_received_invite The local user ID pointer. invitation_id The invitation ID. invitation_host The invitation host ID. sda_template_name The session template name.
var userID = xboxlive_user_for_pad(global.PadIndex); xboxlive_matchmaking_join_invite(userID, global.InviteID, global.InviteHost, global.SessionTemplate);
The above gets the user ID for the given gamepad and then attaempt to join the session that they have been invited to.