What is the ruling on the prayer of the Imam and the congregants if the Imam stands for the third rak'ah and one of the congregants reminds him, so he sits for the first tashahhud, given the differing actions of the congregants between sitting with the Imam, leaving him, or waiting for him while standing?

If the Imam stands for a third rak'ah in a two-rak'ah prayer, he must continue and perform the prostration of forgetfulness (sujud al-sahw) before the salutations (taslim). If he intentionally returns, knowing the prohibition, his prayer is invalidated. As for the congregants (ma'mumoon) who followed him in returning, their prayer is invalidated if they did so intentionally, knowing the prohibition. Their prayer is valid if their return was due to ignorance or forgetfulness. The prayer of the congregants who remained standing or separated from the Imam is valid.