Prophet Muhammad said: “Whoever prays four (Rak`ahs) before Dhuhr and four after it, the Fire will not touch him.” The Sunnah prayer before Dhuhr is between the Adhan of Dhuhr and the obligatory prayer, and the Sunnah prayer after Dhuhr is from after Dhuhr prayer until the Adhan of `Asr.

If the Adhan for Fajr is given while you are praying Witr, you should complete your prayer and there is nothing wrong with that.

There is nothing wrong in offering some Nafl (supererogatory) prayers in congregation, but this should not be a regular habit such that every regular Sunnah prayer is offered in congregation, because that is not prescribed.

The two Sunnah Rak`ahs of Fajr are to be offered after the Adhan for Subh and before the obligatory prayer. They are called Sunnah of Fajr or the Sunnah of Subh and they are among the twelve regular Sunnahs.

The time for Witr prayer ends when dawn comes. However, it was narrated from a number of the Companions that there is nothing wrong with praying Witr after the Adhan of Fajr until the Iqamah is given.