As you probably know, you can restrict access and permission to Office document using Information Protection.
Well, until now, you had to individually add each and every users using their email address.
However there is situation where you can add each users because you may not know or the shared document needs to be protected but with a wider scope.
Good news you can now use the domain name to restrict access and permission to Office documents.
To do so, open the document pane to access the Protect document\Restrict access option
Then when selecting the Restricted access you get the access and permission configuration pop-up; if you see the message including “Specify users by email addresses or domain” then you can use the domain name, otherwise the feature is not yet deployed on your tenant
Include the domain name between quote – like ‘@mydomain.com’ – or you will get an error message saying the email address can not be validated
NOTE deployment is underway for preview tenants and GA is expected to start in early February
Then when opening the document., any user with an email/sign-in address using the defined domain will have access with the defined permissions.