What should a latecomer do if he has missed two rak'ahs and is then joined by worshippers after the Imam is excused, and the imamate transfers to him?

He prays the two complementary rak'ahs and sits for the tashahhud. If he performs the first tashahhud, it is better for him to invoke blessings upon the Prophet and motion for the congregation to sit until he finishes and leads them all in the salām. He then stands to complete his prayer, and they wait for him because he stood due to an excuse. He then leads them all in the salām. This is the prescribed way. However, if he were to appoint someone to lead (a substitute), it would be more preferable, but his prayer is valid.