If we reach the masjid for Fajr salah at the time of jamah then should we join the Jamah or first we should pray sunnah and then join the Jamah.
If there is a separate place in the mosque for praying sunnah away from Jamah and in case of offering sunnah there is hope that one would find second rakah or last qadah then one should join salah after offering sunnah; otherwise join Jamah without offering sunnah. It is better to offer sunnah after Ishraq time.
كذا فى الدر والرد (كتاب الصلاة باب إدراك الفريضة 2: 510، 511 ط مكتبة زكريا ديوبند)
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband