What should a latecomer to congregational prayer do if he catches the Imam in prostration but cannot see the Imam, is it permissible for him to follow the congregants?

It is permissible for the congregant to follow the Imam by relying on seeing him, or hearing his voice, or seeing the congregants behind him, or hearing their voices. The congregant's following of an Imam whom he does not see is valid, by relying on seeing the actions of the congregants behind him. There is no impediment to rising slowly from prostration to ascertain whether the Imam has risen for the first Tashahhud or for the sitting between the two prostrations.