Being a grand or magical gathering only opens up certain activities for you (such as the Wishes Cruise, AK Safari dinner, etc). When you make your reservations, MS will get you a MG number. If others will join you in the "gathering" they should provide the number when they make their reservations.
We've been doing a grand gathering every Oct. because we like the Wishes Cruise and it's a great time for all of us friends (and some family) to get together.