Answered by Shaykh Faraz Rabbani
I have seen people arriving after a prayer has been finished joining a latecomer who himself is making up rakats that he missed. is this permissible?
Answer:

Walaikum assalam,
In the Hanafi school, it is not valid for the latecomer to lead others in prayer, and thus the prayer of those who follow behind him is invalid. [Shurunbulali, Maraqi al-Falah; Ibn Abidin, Radd al-Muhtar]
And Allah alone gives success.
Wassalam,
Faraz Rabbani
