The help page https://www.bt.com/help/account-and-billing/usernames-and-passwords/managing-account-roles/what-s-th... says
Follow these steps to Create a BT ID - make sure you select 'I do not want this to be the Primary BT ID for this account' when you sign up
However, when I try to do this I cannot see this option to select on https://my.bt.com/appsprofile/registration/profile.do
As I am able to select such an option the account number is rejected. Presumably this is because a Primary BT ID cannot be created for it, because there is already an Account Holder.