Google Classroom participants are restricted to the domain. This means that in order to use Google Classroom you must be signed in with your school Google Apps for Education (GAfE) account and anyone in your class has to have the same GAfE account. For example, my domain is @fresnoteach.org. Others who have the @fresnoteach.org email address are able to join my Google Classroom classes.
You can use Google Classroom with others who have a GAfE account so long as the Google Apps domain manager whitelists the other schools domain. When I go to a school to do Google Classroom training, I ask them to please whitelist my domain so their teachers can use my Google Classroom.
Whitelisting must be a mutual agreement between the schools. Each school must whitelist the domain of the other school. If you had your school whitelist my domain @fresnoteach.org you would still not be able to access my Google Classroom training class. We would have to essentially “shake hands” where I add your domain to my list of approved domains.
In order to whitelist a domain you have to have access to the Google Apps admin console.
Under “View configured whitelisted domains” click the “Edit” link.
My experience is that the updates take place instantly. It is possible it could take up to 24 hours for the changes to take place. When whitelisting does not seem to work it is usually because the checkboxes were not checked or the domain was misspelled.
Note: You can NOT whitelist a Gmail account. Whitelisting is only available for GAfE accounts.