You have to have a minimum of 8 people in your party (not including any children under 3) to become a grand gathering. You also must have a minimum of 80% of your party staying on disney property. There are special activities you can do (for a charge), and they will try to get your rooms as close as possible to the rest of your party. If you go to disneyworld.com and type in "grand gatherings" in the searchbox, it will tell you about the grand gathering events. There are 4 of the events, you can also order t-shirts if you'd like.
Have fun planning!
MeredithJ