You have to have at least 8 people in your group to be considered a "Grand Gathering". Note that children 2 and under don't count as one of the 8 (I assume it's because you don't officially pay for them).
We did a GG 2 weeks ago. We had 15 in our group. We did 3 of the GG experiences, which were all very good. Especially the safari and the magical fireworks voyage (this one was my personal favorite - highly recommended). Other than the experiences, there isn't much of a benefit. Actually, we found that if people wanted to do separate things, it got a bit confusing on Disney's end. They messed up part of our groups' reservations based on something another person did.
All in all, it was worth it. We had a great time together!