When adding a guest
DCL asks 3 questions:
1) Is there bed space available in the room? (Can't add a 4th person to a 3 person room)
2) Is there lifeboat space available in the lifeboat that room is assigned to? (If the lifeboat is full due to the other rooms being full, you can't add someone, even if there is a bed available).
3) Is there space in the appropriate kid's club? (if applicable - there is a finite number of kid's allowed in each kid's club level. Once that number is reached no more reservations may be made with a child that age).
If the answer is "no" to any of these questions, then you cannot add an additional person to the room. It may be possible if the "no" answer is to question 1 or 2, you can change rooms, but it may mean paying for a higher category to do so.
Another thing, the new guest will pay the prevailing rate, not the price they would have paid if they had been on the reservation to start with, if the prices have changed.