What happens when you open a new line on your AT&T account?

My family has an AT&T account with 3 other phones, they want to open me on a new line and I was wondering what they do like do they have iphones for free when you open a line or do you get it for 99 dollars? I wanted to know what will happen and what are my chances of getting an iphone 5

If they open a new line you can get the iphone 5 for $100 at at&t.