The process of admin registration, unlike user registration, is rather more earnest and implies greater responsibility. Which is why it includes an intermediate step for a higher security.
In particular, a potential admin will be asked to go through the sign up flow, in the process of which – get an account ID and pass it to an existing admin with the account manager right. An existing admin should add an account with the received ID. A particular person will be redirected to the next step of the sign up flow.
It is impossible to get an admin account without the approval of the existing admin who has the account manager right.