You can add more than one person, other than the Captain, onto your other three ships. All of their scores will stack with that ship's captain's score. There are two different ways to do this. The first method is at the "Open Sea" screen (where you can see all of your ships), get close to another one of your ships and a swap cargo icon will appear. Go into the swap cargo menu. There is an option to switch captains. Under this option, there is a screen that has four slots. One of them is the captain of that ship. Click on one of the other three slots, then assign passengers to each one. After doing this, they disappear from your passenger list. You will need to get more party members at the tavern if you want to fill up all your ships. Don't sell the ship with the other three crew members still on it. You will lose anyone that is still on board. The second method is to capture a ship and do it when you assign a captain to the captured ship.