Q. If a person is praying nafl salah and someone joins them and prays as jamaat, does the person leading have to make intention of leading that person?
A. The performance of Nafl Salaah in congregation is permitted if there are three or less followers behind the Imaam. The Imaam does not have to make a specific intention of leading the followers unless there are female followers in the congregation. If there are female followers in the congregation and the Imaam does not make an intention of leading them in Salaah, the Salaah of the female followers will be invalid.
Nonetheless, the followers, male or female, must make an intention of following the Imaam in the Nafl Salaah performed in congregation. (Durarul Hukkaam 1/120)
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa
themufti.com