What is the ruling on vocalizing the recitation in a supererogatory prayer when the person praying supererogatorily leads those performing a fard prayer? And does the ruling differ based on the rak'ah or the prayer (such as Maghrib, Isha, or Witr)? And what if he began reciting and then someone performing a fard prayer joined him; should he repeat Al-Fatihah or continue his recitation?

It is permissible for someone performing a fard (obligatory) prayer to be led by someone performing a nafilah (supererogatory) prayer, according to the preponderant opinion. The evidence for this is what is narrated from Mu'adh ibn Jabal (may Allah be pleased with him), that he used to pray 'Isha with the Messenger of Allah (peace be upon him), then return and lead his people in that same prayer. If the person praying a supererogatory prayer becomes aware that someone is being led by him, he is not required to vocalize the recitation, because the Sunnah for supererogatory prayers is to recite silently in all cases. Nor is he required to repeat the recitation; rather, he should complete his prayer.