أحسن الله إليكم Every monday we meet with brothers in a mosque of ahlul bidah (mufassiqah) and do muraja‘a of some Ahadith we learned. But some of the brothers dont pray behind the people of the mosque or pray with the Intention of nafl and repeat the salah afterwards alone. What’s the hukm?

May Allah subhanahu wa ta'ala reward you for being keen on studying. However, I do not know what you mean by mufassiqah or which party this is. Either way, not praying behind the people of Bid'a is incorrect, and the Sahaba (the companions themselves) prayed behind the Khawarij, and the Khawarij are from the people of Bid'a. Of course, we speak generally; we're not talking about a Bid'a that is disbelief.