When you first open the mobile GroupMe app, you are asked if you would like to sync your contacts. If you opted not to sync your contacts but have changed your mind, you can sync at any time from the native Settings menu (gear icon).
- Go to Settings.
- Find Contacts and make sure Sync Contacts to Server is on.
You can also sync your contacts through the GroupMe app Settings. Make sure the Sync Contacts toggle is turned on.