Hello Paresh,
Well, English will never atop surprising the learners I guess! Regarding your question, both can be correct, it all depends on the context.
Pay attention to these two sentences extracted from Longman dictionary:
1. I’ll raise the matter at the next meeting.
2. She said that Mr. Coleby was in a meeting.
“In a/the meeting”, which is used more commonly, addresses a literal room in which some people are discussing some issues.
“At a/the meeting” is generally about the concept of a meeting, but not precisely a room in which people are talking about a particular subject.