What is the ruling on a man praying Fajr in congregation, then praying it again with another man who missed the the congregational prayer, in order to gain its reward?

There is no harm in someone praying with another person who missed the prayer after Fajr or Asr, because the rule is that there is no harm in performing a voluntary prayer that has a reason during times when prayer is forbidden, such as the greeting of the mosque (tahiyyat al-masjid), prostration of recitation (sujud at-tilawah), and the prayer of seeking guidance (salat al-istikharah), and giving charity to one who missed the prayer.