Well you can just activate new lines under your account for them or go prepaid. You won't have any issues or lose your number. Can we have more details in case I'm not understanding what you're saying?
Guessing the web interface keeps reverting to the OP phone and account info as she tries to add new lines.
I recall a conversation a while ago indicating you had to , in effect, choose the new number twice - once to select it, then again to modify it.