Hi Nancy,
Thanks for reaching out! That error usually happens if GroupMe can’t verify your number for SMS.
Make sure you’re using the right country code and no spaces.
If that’s still not working, sign up with your email instead at groupme.com/signup and add your number later.
If it keeps acting up, reach out to GroupMe Support with your number, country, and a screenshot. https://support.microsoft.com/en-us/office/how-do-i-contact-groupme-customer-services-906e6ade-807e-46b3-ba6c-510cd01a0647