If you have never set up Account Administrator relationships, you can no longer create new ones. Instead, use our new Account Access feature (more info).
If you want to give someone access to your hosting account but not all of your other account information, you can assign them as an Account Administrator. This lets them log in to their customer account and manage only the aspects of your hosting account you specify.
The process for assigning Account Administrators is (your actions are in blue, the new Account Administrator's are in green):
Creating a New Account Administrator sends the recipient a request email they must accept before you can assign them to your hosting account.