If the ship is sold out people are put on a wait list. Sometimes people put a deposit on a cruise and then decide at final payment time they aren't going. This will free up cabins which will then be sold to the people on the waitlist. Groups can reserve a block cabins and then if they don't sell them all they'll release them, also at final payment time (at least this used to happen, not sure if they can still do it with some of the new rules the cruise lines have instituted).
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a lot depends upon what number they are on the wait list. Are they waitlisted for a specific cabin or cabin catagory or will take anything that comes along? All these factors contribute. Usually what happens is that someone will make a deposit and then cancel before final payment due date (90 days prior to sailing) that will free up some cabins, also large travel agencies buy up blocks of cabins, they have a specific date that they must sell those cabins by or they must release them back to the cruise line.
As Lisa said, general waitlist is best and the number on the list can give you an idea if you will make it or not. Usually under 20 often make it in the off-season. For high season I just would hate to guess.
I'd also suggest that your friends go to a large local travel agency, if they haven't. As LisaK said, TA's buy up blocks of cabins. Maybe their local agency is one of them!
I find it hard to believe that this cruise is sold out, unless a huge group has booked most of the ship!
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