You can also use a bank debit card, but not a cash card (i.e., prepaid Visa card).
It is my understanding that if you use cash, you have to wait until after you board and go to the purser's desk to establish your sail and sign account with cash. If they use a bank debit card instead of cash, they will save having to waste part of their cruise standing in that long line the first day.
If they have minor children in their group, they do not have to fund their card. This will keep the children from running up a tab or losing their card. The children's portion of tips will be added to the adult's Sail and sign account.
Been there - done that:
2005 Holiday, Sensation, Conquest,
2006 Conquest, Celebration, Holiday,
2007 Freedom Grand Med, Holiday
2008 Fantasy & Sensation,
2009 Fantasy, Holiday & Dream Grand Med
2010 Fantasy and B2B Elation
2011 Monarch of the Seas
2012 Booked - Breeze from Barcelona 12 days