If we are late in namaz and imam has completed the prayer, and there are 4,5 people whose jamat has missed then we do our own jamat then it is necessary to say takbir again?
بسم الله الرحمن الرحيم
Yes, it is sunnah to say takbir that time too. However, it should be noted here that according to Shariah it is makrooh to establish second jamat in a mosque where already salah has been performed with jamat. However, if jamat is missed by chance then it can be performed with jamat in Imam’s room with his permission, or in the outer portion of the Shariah mosque.
Allah knows Best!
Darul Uloom Deoband