Is it permissible to complete the prayer with the congregation in the last rak‘ah, or must one wait until its end?

If you find the congregation in the last rak'ah before they rise from its ruku' (bowing), you must join them and complete your prayer after the Imam's taslim (salutation of peace). However, if it is after rising from the ruku' of the last rak'ah, if you do not hope for another congregation, then join them and complete your prayer after the Imam's taslim. If you do hope for another congregation, you may wait.