If a follower (Muqtadi) forgetfully omits a necessary act (Wajib) while praying in a congregation behind the Imam, what must they do?

They must perform Sujud al-Sahw independently at the end.
Their prayer is invalidated and they must repeat it.
They are not required to do anything, as their following of the Imam absorbs their mistake.
They must break their prayer and perform a new Wudu.