Signup from the admin control panel is also the one and only way to create accounts that are unavailable "from the street", regardless of whether they are tagged available or unavailable for signup, for instance admin and technical support accounts.
To sign up a new user from the admin control panel:


If the list doesn't offer desirable plan, check if it is available for signup.
When you are through with the Signup Wizard, account will be either created or assigned for activation.