What should the worshipper have done when someone followed him in prayer, thinking it was the Maghrib prayer, while he was performing a supererogatory prayer?

It is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafl (supererogatory) prayer, just as Mu'adh ibn Jabal used to pray the 'Isha prayer with the Prophet, peace and blessings be upon him, and then return to lead his people in prayer, where it would be a supererogatory prayer for him and an obligatory prayer for them. Similarly, it is permissible for a worshipper to begin their prayer alone and then become an imam when others join them; this is valid for both fard and nafl prayers. In this case, the imam has the choice between audibly reciting (jahr) and silently reciting (israr). However, if one intends to be an imam from the beginning, then they should recite audibly.