You can usually add a third or even a fourth person to your cabin at a later date as long as the room is large enough to accommodate the extra passengers. If the cabins on a cruise ship don't get reserved by the normal deadline, the cruise line may extend the "early-bird" rate.

Adding someone to a cruise at the "last minute" (within 2–3 days of sailing) is extremely difficult and often impossible in 2026 due to strict international manifest laws. Cruise lines must submit a final passenger list to government border authorities (like the U.S. Coast Guard or EU Customs) usually 48 to 72 hours before departure. If you try to add someone after this "lock-out" period, the system will not allow the update. Even if you are outside that window, you can only add a person if the ship hasn't reached its maximum lifeboat capacity—which is different from cabin capacity. If the ship is "sold out" on safety capacity, you cannot add a third or fourth person even if your room has an empty bed. If you can add them, expect to pay the current prevailing market rate, which is often much higher than your original booking price, plus additional port taxes and fees.