ANSWER:

If you were with the imam when he made that mistake, you should offer it after you finish your own prayer 

But if the mistake was done before you joined him, you don’t need to do anything

QUESTION: What do I do when I join a salat late and once the iman finish he has a sujud sahw to do . Should I do that with him before completing my missed rak’ats or do it once I finish my salat