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.
Hello, thank you for reaching out to Microsoft Q&A.
May I please confirm whether you experience the same problem when using a different browser, device, or network?
If the problem only occurs on a specific device while connected to a specific network, please open an incognito/private browsing window in any web browser and connect your device to a mobile data network.
Then, paste this link into the incognito window to create a new GroupMe account using your phone number:
https://web.groupme.com/signup
Enter your email address and, in the field where you see your phone number, select the correct country code and type in your phone number.
If you receive a message stating that your "phone number is not verified or valid" when entering your number, it means that GroupMe is unable to register your number. This may be because the number belongs to a country that is not supported by GroupMe, or it is a type of number (such as VoIP or toll-free) that is not eligible for registration.
Reference:
How do I register my phone number on GroupMe?
Let me know if you still need more help.