What carrier do you currently have, and what company did the phone orginally come from? The phone is usually tied to the company unless you can get them to unlock it.
If you have Verizon, you should be able to share data on a family plan, an additional line usually costs $9.99 plus the taxes (comes to about $15 a month total on my family plan), however any changes to your family plan will mean you lose unlimited data if you already have it.
You might be able to put that iphone on a prepaid plan, IF you can get it unlocked and it's comptible with the nextwork of your new chosen carrier.