Is it permissible for a group to join a ma'mum (one praying behind the imam) and take him as their imam in prayer?

Question:
May Allah reward you. This questioner says: A man prayed as a ma'mum behind the imam and only caught the last rak'ah. When the imam said the taslim, the ma'mum stood up to complete the rak'ahs he had missed. While he was standing for the second rak'ah, the mosque congregation entered and stood behind that worshipper, taking him as their imam and praying behind him, even though he had previously been a ma'mum. Is their prayer valid? Please inform us. May Allah reward you with good.

Answer:
Their prayer is valid, if he was aware of them, if he was aware that they were behind him, and he prayed considering himself their imam, then their prayer is valid. However, it is better for them to pray in congregation by themselves and leave the one who is making up the prayer he missed. It is better for him to intend that what he is doing is a voluntary prayer, complete it, and then join them. This is if he caught less than one rak'ah with the first imam, less than one rak'ah. But if he caught one rak'ah or more with the first imam, then he has caught the congregation, so he continues and completes his prayer, and they pray in congregation by themselves.