This happened to me a few days ago. The messages went under the "message requests" section (which i don't think you get notified of). When I told a couple people in the group about it, they hadn't even noticed...

That said, if you know who is in the group and happen to be close with any of those people - I would recommend telling them about it so they can report the message if it did go through. From what I can tell that's the key to IG taking action to delete their profile. So far I think my scammer only leaked to that one group of 6 people before their profile was deleted (i know they have at least another profile, but still think getting their ish deleted is a big step in them moving on)