بسم الله الرحمن الرحيم
If someone joins the jamat at a time when the imam had already started qirat then the person should stand silently after joining the salah and should not read Sana. And if he joins the jamat at a time when the imam had finished reading qirat in the first rakah and bowed down in ruku then if he hopes that he shall get the imam in ruku after doing takbir tahrimah and reading Sana then he should read Sana and then join him in ruku, but if he fears that the imam shall raise his head from the ruku then in such a case he may leave Sana and join the imam in ruku. And if someone is masbuq i.e. he has missed one or more rakah then he should join the imam immediately and when he stands for completing his missed rakahs he should read Sana first and then do the other things.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.