What about someone who joins the congregational
prayer late and has to make up the rak’aat missed, and
finds that the Imam makes sujood al-sahw? Should he
join in the sujood or stand and continue his prayer and
make it at the end?

If the Imam makes the salaam and the follower stands up to
complete what he missed, while the Imam has to make prostrations
of forgetfulness after the salaam, the follower should return back
and make prostrations with the Imam if he has not completed
standing upright. Otherwise he should not return. He should
complete his prayer and after it he should make the missed
prostrations. The evidence for this is the same as presented in the
discussion for Forgetting to sit for tashahhud between the second
and third raka’ah. (al-Mughni 1/697)