What’s the difference between i’m in that group chat and i’m on that group chat

The difference lies in the use of the prepositions- in vs on so “in” is used to express the time, place, or surrounded by something.
While “on” is used to express where the object is, a particular place, over a thing.
So for this instance, I’m in the gp means that you’re included in it. I’m on indicates that you’re using it. This is just my opinion to how I see it
Hope this helps :)