These are more or less interchangeable and both are correct. The connotations are a bit different. I would say "in a meeting" every time. However "at a meeting" also is correct For example: "In a meeting" implies that you are a part of the meeting. This is definitely more common. "at a meeting" implies that you are physically at the meeting, but that you may not be taking an active role in it.

Although they can be interchageble they both have a correct form . For example if you were present at the meeting ...then you would say...I was in the meeting we discussed ....but if you were referring to a meeting you would say ..At the meeting they discussed ....

For example: •The situation was reviewed at a meeting of the Standing Veterinary Committee last Monday. •Sorry, I am in a meeting now.