A Grand Gathering is a group of eight or more people (ages 3 or older) who are staying at Disney Resorts. There are certain activities which may only be attended by people who are members of a Grand Gathering.
There are two "Eight and 80%" rules which apply.
A GG may include people who are not staying at Disney Resorts. However, at least eight of the members must be staying and also at least 80% of the people must be staying. If, for example, you had 12 people total, 80% of that is 9.6 so at least ten of the people must be staying at a WDW Resort.
If, for example, you had a GG of 50 people, you would have to have at least 40 of the people staying on property and could have ten communting or staying other locations.
Likewise, each GG activity uses the same 8/80% rule. If you had the 50 person group and were to have an activity for only 20 people, at least 16 of the people attending that activity (80%) would have to be staying at a Disney Resort.