The experience has varied depending on who is the onbooking agent, but Leslie, who's been on the Magic, has repeatedly allowed us to book rooms for friend and family who aren't currently onboard, for a future cruise that we're booked on without any problems. A straight onboard booking is superior to the friends and family offer, since it gives you the full onboard booking benefits, along with the ability to change your sailing date in the future. With the friends and family offer, you only get the OBC but no discount, and if one of you changes to a different date, then you lose those benefits.
You can either reserve your own room in advance, then make sure your family has your reservation number and information, and they can try to rebook it for you at that time. You will have to pay the full 20% deposit though if you reserve it in advance. Or, since they are family and you might trust them, they can reserve the room for you while onboard, and pay the deposit using your credit card information, or pay the deposit for you to be reimbursed once they're off the ship. The first option, where you do everything in advance and they try to rebook is the easiest for them, and ensures you of getting what you want. Otherwise, they'll need you names, birth dates, addresses, and all that other information.
Leslie has told me that we could book unlimited cruises for ourselves, but the number of cruises we can book for others is limited to the number of people in our current sailing party. So they should be okay. If you don't plan on sailing on the same cruise with your family member, one or both of you can use a dummy date when rebooking, so you are technically on the same cruise as them, but either of you can change once booked without any problem.
