My question is that if someone is busy with Sunnat or Nafl prayer when the congregational prayer is about to begin, then what should he do? Must he perform Salam and join the congregational prayer or must he complete his Sunnah or Nafl?
الجواب حامدا و مصلیا
If the congregational prayer begins while one is performing the Sunnat of the Fajr prayer and he is hopeful that he can complete his prayer before joining the congregational prayer, then he should complete his Sunnat. As for Sunnat or Nafl prayers other than the Sunnat of Fajr, if one has performed two Rakats (or less) and has not started the third Rakat, then he should perform Salam after the first two Rakats and then join the congregational prayer. But if one has already started the third Rakat, then he should complete all for Rakats and then join the congregational prayer.
And Allah knows best.