Is the Imam the one who missed the first congregational prayer, or the one who stood up to pray a supererogatory prayer with him, if a man entered the mosque after the Imam had said the Taslim and one of the worshippers stood up to pray with him?

The first [option] is for the person who missed the prayer to lead it, because he is performing an obligatory prayer while the other is performing a supererogatory prayer. Although it is permissible for someone performing an obligatory prayer to follow someone performing a supererogatory prayer, it should be avoided to steer clear of the scholarly dispute regarding the invalidity of such following.