To prevent unauthorised claims from unknown sources, by default, only email addresses sharing the same domain can be added to Grab for Business.
We understand that there are certain groups that require multiple email domains, therefore the ability to enable this feature is done on a case-by-case basis. Do note that group settings are locked upon creation. Even after admin permissions have been granted, only new groups are able to add users with other email domains.
To enable this function, contact us at firstname.lastname@example.org with email subject Multiple Email Domain, together with the reason you are requesting to add users with other email domains to your group.