[Slartibartfast]

If I remember correctly, when you are at the page that asks you to sign in or choose the account name (i think it has whatever the registered main account is and a default option that is always there called "other user") there is a link on that page called "manage accounts". Click on that and you will need to sign in under the main account name. You can then create a sub-account in your name.

I haven't done this myself, but I have Yahoo Broadband on my home PC and I think this is how you do it.

Lemme know if it works!