I have heard of other families getting together and splitting room costs...It's a great idea for those who have more than four in a family. You may want to check the cruise meet board to find others going at that time.
When we went...we were only a family of four...so we never asked about getting additional wristbands. What are the ages...I think not to long ago some one reported not needing a wristband for an infant...but I'm not 100% sure on that....another thought...Is the WHOLE family of five wanting to use the pools and slides...or do they have other things at Atlantis there wanting to do? You only need the wristbands for the pools, slides, lagoons and private beach. You can pay for admission to the dig for one person if needed...and you can roam around the hotel and casino along with the shopping area all you want....without a wrist band.