I am actually in this same predicament. I am currently on a Family Shareplan with my mother (I'm a college student). We used to have 2 other lines on this plan YEARS ago, but they were deactivated a while ago. When I spoke to a CSR 2 days ago, he said I could reactivate one of those lines and then that line would be eligible for an upgrade. I'm going to visit my local Verizon store tomorrow to confirm this, but I think it should be as easy as "I want to add a line and I want to put the iPhone 4 on it.' Okay, now you owe Verizon $200 for this phone." And from what I understand, it should only cost an extra $10 a month + the data package to do this.