Absolutely yes. The credit card used for booking has nothing to do with securing the Sail & Sign accounts. They won't know the credit card used for booking when the accounts are set up. Each person can secure their account online or in person during the check-in process.
Each person will have a seperate folio of charges. They can also change the credit card, or use another method of payment to secure the account by going to the Info/Purser's Desk.
Best regards from another 1993 Carnivale cruiser.