A feature that allows users in the U.S. to participate in GroupMe chats via standard text messages, without needing the app or internet access.
Sorry to hear you're having trouble.
If there is no option to 'Try Another Way' when requesting your pin then you may want to try contacting GroupMe support via this link https://support.microsoft.com/en-us/groupme/how-do-i-contact-groupme-customer-services
However, please note that they are aware of log in issues with the app at the moment. While waiting to contact support I'd like to recommend reading through this post https://support.microsoft.com/en-us/groupme/groupme-log-in-issues