UPDATE Google Admin - Whitelisting Domains

There are two situations that I have run across where a G Suite for Education School would need to whitelist a domain (or subdomain).  These are:

1. If your students and teachers are on two different domains
2. If you are in a situation like me - as a support organization I want to provide training to teachers that are in their own Google Apps district

It is quite easy to do this - it just has to be done by a Google Apps for Education Administrator. There are THREE parts to this, make sure all of them are set for it to work properly!

Part 1 - add the domains

In the Google Admin Dashboard, click on Domain (if it doesn't show, click on the More at the bottom)

Once in the Domain management area, click on Whitelisted domains-

Click on Add new -

1. Type in the domain to whitelist, then
2. Click on Add Domain to Whitelist -

Part II - Set Classroom Settings

Go back to the Home level of the Google Admin panel - click on the Apps icon:

Click on the G Suite Core Service apps:

Click on Classroom:

Click on Class Settings:

Select the options you desire
1. Select if you want people from other domains to be able to join your domain's Classrooms
2. Select if you want to allow your domain accounts to be able to join Classrooms in other domains

Part III: Double-check your Drive Settings

Since Classroom utilizes Drive, you need to make sure that the domain connecting with Classroom doesn't have the strictest permissions - in fact, a new option for whitelisted domains has been added as an option to choose.  To check this, click in the breadcrumb at the top to return to Google Apps.
Then, click on Drive:

 Click on Sharing settings:

Check to make sure the two boxes under the Whitelist settings are checked.  If they are not, your sharing may be off (top choice) and you will MINIMALLY need to change it to whitelisted domains.  (If you have sharing set to ON the whitelisted domains are automatically ok) -