What is the ruling regarding someone who missed the 'Isha prayer, then led a congregant, and then another congregant led him in the last rak'ah?

It is permissible to follow a latecomer (masbuq) if he stands up to complete his prayer, because he has become an individual (munfarid). He should intend to lead (imamah) if someone else follows him. It is not permissible for a person to be both a follower (ma'mum) and an imam at the same time, according to scholarly consensus (ijma'). So, if a man joins you while you are a follower, his prayer is not valid, and he must repeat it. However, if he joins you after your imam has given the salutations, his prayer is valid.