When you want to add another person to a reservation
DCL asks 3 questions:
1) Is there another bed in the room? (can't put 4 people in a 3 person room).
2) Is there space in the lifeboat assigned to that room? (It's possible for a lifeboat to be full, even if there's available space in the room).
3) Is there space in the appropriate kid's club? (if applicable)
If the answer is "no" to any of these questions you won't be able to add another person. If the answer is "yes" to all you can add more people all the way up to the day prior to the cruise (as long as the answers all remain "yes").
If the answer is "no" it may be possible to change your room (extra cost) to one that the answers all become "yes".
And, finally, the new person will pay the prevailing rate, not the cost they would have paid had they been on the reservation from the beginning.