Hi, @bella8910! Most our plans are single/family plans, it means that the same single plan turns into a family plan when you link another account to your account. Thus, the plan just changes the amount to pay and how the payments are taken.
Before you link the accounts and create a family plan, the new line must be a single line first; this is due to the fact that we're unable to link interrupted account or account without service. However, you'll be receiving a prorated credit to the primary link for unused days of service.
You can activate the phone online (https://activate.boostmobile.com/primary/activations-home?intnav=UtilNav:Activate) and add the line to your plan. If you need further assistance, you can send us a Private Message and we'll be able to help you.