Both IN and AT a meeting are correct, but are not used interchangeably.
IN a meeting is used when referring to “active involvement in the meeting”:
- Can I talk to Mr Bean, please?
- I’m afraid he’s in a meeting. Can you call back later?
(Mr Bean is busy: he is taking part in the meeting.)
AT a meeting is used when referring to the physical location:
- Is Mr Bean in today?
- No, he’s at a meeting all day.
(Mr Bean has left the office and now is at another place.)
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