Users get "sorry this site hasn't been shared with you", but they are definitely members of the site. I just added a test user to the same group, ran IE as the test user, and was able to log in. But all of the "real" users still get the error. I, as the administrator, can log in fine. I made another user an owner and they still can't log in.
However, there is a subsite of the main site collection and the users CAN access that. I'm at a loss!