All Organization Account employees are allowed to invite employees of same domain as Organization Account domain defined in My Account settings. If you want add employees of any other domain, update domain of the Organization Account in Account Settings. Only Super Admins of the Organization Account can update the domain. For more information, refer to Updating Domain of the Organization Account.
To add employees to the Organization Account
Select the Contact Type as Employee.
Enter User Email ID.
Make sure that domain of the entered Email ID is same as the Organization Account domain defined in My Account settings.