You'll also receive your own personal Grand Gathering number. Everyone in your party will have to add this number to their reservation. If everyone is staying in a villa with you, there shouldn't be a problem. But if some people are staying in other resorts, they'll have to add the number to their reservation.
There is also an 80/20 rule. A minimum of 80% of the people must be staying on property, and 20% can be staying off property or be locals. If any of this is applicable, GG will ask for more information about the off property people.
I set up a Grand Gathering for the Pirate's Cruise last October for 16 people. We had a blast!