The first questions I always ask when I hear this are:
- What kind of account did you use to create the classroom? Was it GSuite for Education? Personal Google Account?
- What kind of account are students using to join the classroom? GSuite for Education account issued by your school? Personal Google Account?
Classrooms Created with GSuite for Education Account
By default, every GSuite for Education domain has classroom set so that only accounts on the school domain can join classrooms on the school domain. Every school or district that uses GSuite for Education has a Google Administrator who provisions accounts, chooses settings for GSuite services, etc. In most cases, GSuite Admin choose to not change this setting.
However, the admin can choose to allow out-of domain users from other GSuite domains, or from any domain to join classrooms on the school domain as well, as well as allow the users on the school domain to join classes on other domains. If the user is from another GSuite domain, their Google Admin will need to enable users on their domain to join classes on other domains as well.
So, if the person joining your classroom is from another GSuite domain or has a personal Google account and you are getting this message, you will need to contact your domain's Google Admin to determine whether they will allow out-of-domain users. Please see this reference for assistance contacting your Google Admin.
Classrooms Created with Personal Accounts
All personal Google accounts can join Classrooms created with personal accounts. However, if students with GSuite accounts try to join a Classroom created with a personal account, they will receive the error message. At this time, GSuite for Education accounts cannot join Classrooms created with a personal account.
Keep in mind, that if you are creating a classroom with a personal Google account, you cannot use this classroom with students in a school setting. For one, it is against Google Classroom TOU.
For another, Google Classroom within the GSuite environment meets the requirements set forth by federal privacy laws like COPPA and FERPA. Google Classroom when used with personal Google Accounts is not compliant with these laws, and therefore cannot be used with children in schools.
I hope this helps to clear up any confusion if you've been having trouble adding students to your classroom!