We live in Fort Lauderdale so we cruise over to the Bahamas quite often. Grand Bahama is closer and currently has a high speed ferry going over on a daily basis, and there is a two night cruise that goes out of West Palm Beach. There used to be a day cruise ship from Fort Lauderdale, but unfortunately that stopped - as did the two night cruise ship to Nassau.
It sounds like your friends will be in Atlantis though - which means you need to get to Nassau. There are weekend cruises that spend a full day in Nassau, but you would have to plan on three nights - two full days (Carnival, Royal, Norwegian, and Disney). Otherwise you would have to fly. Chalks was a great experience, but as a previous poster mentioned, they are no longer flying. Spirit offers inexpensive flights but it may be to Grand Bahama. United has direct flights over and I believe American does too. Jetblue and Airtran used to go over, but not sure they still do and don't know whether any of the other major airlines go over directly? Bahamas Air is often pretty economical as well. They left our luggage in Lauderdale once because it was the holidays and the Bahamians were bringing back a lot of goods - not enough room for our luggage - but this tiime of the year it shouldn't be an issue. Flight is an easy flight and often economical - the worst part is the taxes.
If you fly you need a passport, if you cruise you just need a passport card. Would make a nice short trip. Laker's Airways used to fly over there for daily casino trips, but they haven't done that for years.