"in or at" a meeting

It depends on context.

For example, if you are in the meeting space and are participating in the meeting, we use "in."

In certain situations, you might use "at," but that indicates that perhaps the meeting is on break and you are outside of the actual meeting space.

For example, let's say that I attend a conference for Wizard professionals and I call my sister.

I might tell him: Hey darling, I'm at the conference.This means that I am at the venue where the conference is taking place. I am not, at this moment, actively participating in the conference.

Lets further say that while attending the conference, I meet with several other Wizard professionals. During this meeting, my sister calls me on my cell phone. I might answer it* and say: I can't talk right now. I'm in a meeting. In other words, I am actively participating in the proceedings.

You also can say "at" a meeting if you want to emphasize that you are, for example, out of the city attending a meeting.