GroupMe account suspended, how do you find out why?

check the email on the account, spam folder too, sometimes there's a short message with it. usual triggers are getting reported by people in a group, or doing things that look like spam, like adding loads of people at once or posting the same link in lots of groups. brand new accounts get caught more easily. making a fresh account on the same number tends not to work, so the ticket really is the way.

You mostly don't get a reason beyond the suspension message. Raise a ticket through the GroupMe help centre and ask for a review, that's the only route.