Are you for sure, Benjamin? I believe that only lets them add Retail Authorized users. Truly, to create a secondary Access ID it would go like this.
Account Owner (person whose name appears on the bill) would need to create an Access ID (or CONVERT their CTN ID).
Then, I, having a secondary line, would go in and click "REGISTER NOW" for user ID. I would get a message saying "This account has already been registered, wouldyou like to request access from the Acct Owner?" Select yes, so and and so forth. After I finish... Acct owner would have to log in with THEIR access ID and then hover their cursor over the top of the page on the dark grey bar that says "My Linked Accounts" which would drop down a menu that says "Manage Acct Access" and from their, they would see the option to ACCEPT the users access or decline! :)