Each member of the party has to have used their first 3 FPs, but once they do that, any one person (who is linked to the other people) can book for the whole group. Since you don't scan your admission medium (ticket or MB) on the way out, the system has no way of knowing if any particular person is in the park or lounging at the pool. In other words, the system knows that you have entered the park, but not when you have left it. Now, if a person leaves and then enters a different park and thereby notifies the system that they are no longer in the earlier park by scanning their admission medium at a different tapstile, I don't know what happens.