As long as your stateroom that you have reserved is able to accomodate another person you should be fine. Last year, when we booked our cruise for this year while still on board the Magic, we said we really couldn't afford the deposit right then for each of our family members. The lady told us to just pay for as many as we could and we could add the others on later. We were sure to reserve a stateroom that could accomodate the 5 of us, so that there would be no problem adding the others on later.