What is the ruling on the prayer of someone who caught up with the Imam in the last rak'ah, then completed his prayer alone, but then worshipers joined him from behind, so he prayed a fifth rak'ah with them, knowing this, for fear of confusing them, and what is the ruling on the prayer of those who prayed behind him?

Committing a sin by deliberately adding to the prayer necessitates two things: First, repentance and seeking forgiveness, and second, repeating the prayer due to its invalidation by the deliberate addition. As for the prayer of the congregants, it is valid unless they were aware of the addition; otherwise, it becomes invalid if they followed the Imam in it while knowing.