Unlike a hotel room where you can often “show up” with an additional person or even “sneak” an addition person into your room, cruises are different requiring every passenger to be accounted for in advance, even babies. This has to do with coast guard safety requirements in addition to having the required boarding documents.
Not all rooms can accommodate more than two people; rooms that will accommodate more are often limited to exact number i.e. 3, 4 or sometimes more depending on the cruise line. Many of the promotions, specials, deals are based on double occupancy (two passengers). They are often offered on a guarantee basis meaning the room is not assigned until after payment is made and can be as late as day of sailing. During peak periods it is not uncommon for there to be no rooms available for triple or quad occupancy requiring the purchase to two rooms. Sometimes availability can be restricted due to ages of children due to caps on the children programs, especially with Disney Cruise Line.