We didn't use the grand gathering available at WDW when there was 9 & more of us, but I thought they had someone available at WDW to specifically help groups with this? But I'm not sure, maybe someone else will post that is.
But if you wanted to do this yourself, I would check the on-line availability for your party & try to secure some ADR's now. In our case, I actually had a very cooperative group of lady family members going so we had a BBQ & tossed around general ideas, then I followed up with a questionaire with questions like: List you top 3 favorite countries you'd want to dine at in EPCOT; do you prefer character bfast or sit down dinner on MK day, etc. Then I tallied up everyone's response & sent out another "survey" listing the top 3 choices for each question to re-vote on. And I'm sure I sent a final showing the result & for everyone to comment they'd go along with it or if they'd have different plans that day.
But this was before 180 day out ressies so it was easier to get everyone to think about it 6 months out so that when day 90 hit I was able to call WDW-DINE & get all our ressies booked together.
We made it as our own little pre-trip fun, choosing disney names for everyone & since we were celebrating my aunt's 50th b-day, everyone was extra cooperative & we had a wonderful time. Among others we had bfast at Cindy's castle, CM dinner, H&V Fantasmic dinner, R&C dinner, Donald's bfast in AK and then we separated for some of the meals in EPCOT due to different plans & tastes. But those big family meals at WDW are some of my fondest memories ever

But my family was very supportive of my effort to make the trip a good one. Everyone agreed & were appreciative of my efforts to preplan for them.
Hope your group has a great trip.