Offering Salah behind one of the Ma'mums when entering the Masjid after the end of Salah

Q 2: If someone enters the Masjid (mosque) after the end of Salah (Prayer) and finds one of the Ma'mums (people being led by an Imam in congregational Prayer) completing his Salah, should he offer Salah behind that person and make him the Imam after he was a Ma'mum, by standing to his right and tapping on his shoulder to inform him that he has joined Salah with him? Please advise concerning the Ma'mum being an Imam (the one who leads congregational Prayer) and offering two congregations at the same time in the Masjid.

A: It is permissible for a person who enters the Masjid after the end of Salah to offer Salah behind one of the Ma'mums who have joined part of the Salah with the Imam. In this case, the one who was the Ma'mum is now the Imam, according to the most authentic of the opinions of scholars. However, it is impermissible to hold a second congregation before the first congregation is finished. If a person enters the Masjid and finds a congregation, he should join them.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.