I would think you can pray Fajr (or any salat) in jamat up to the same time you could pray it alone. So for Fajr, if you said the first takbeer before sunrise, even if it was 1 minute before sunrise, then Insha'Allah your salat will be considered on time, whether in jamat or alone, even if it goes past the sunrise time after you have started praying.
However, that does not mean that one should purposely delay Fajr (or any) salat until the last minute just because they "can". There are stern warnings in Quran and in many ahadith regarding those who purposely delay and don't make salat on time. This type of situations should only be happening in an emergency, such as oversleeping or some other extraordinary circumstances preventing someone (or a group of brothers) from making salat on time, so they were forced to do it at the last minute. They should still make jamat of course, even if it is at the last minute. I know you are not asking because you are looking to purposely delay your salat, so don't get me wrong here, I am just making a general comment so anyone reading this does not get the wrong idea Insha'Allah.
May Allah forgive me if I said anything wrong. Ameen.
Allah knows best.

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