Interrupting the prayer to join a group prayer

• If someone completed his obligatory prayer alone, then he may join the group prayer afterwards as an optional prayer, but not after Fajr, Asr or Maghrib.
• If he has not made the first prostration of the second subset of Fajr; he interrupts Fajr and joins the group.
• If he had prostrated once in the 3rd subset of Thuhr, 2nd subset of Maghrib or Fajr then he completes the prayer.
• If he was praying the sunnah mu'akkadah (ascertained merit) before Thuhr or the Friday Group Prayer, and then the group prayer or the speech started, then he interrupts after competing the first pair of subsets.