Your teachers/students might be in ‘pending’ status. All you have to do is to activate teachers/students accounts to add them into groups.
To activate an account, here are the steps:

1. Go to [Admin] > [Manage Teachers/Students]
2. You will see the teachers/students are in the ‘pending’ status.
3. Click on the tick icon to approve the pending teachers/students.
4. Now your teachers/students should be ready to be added into the groups.