So I actually did two trips almost back to back: the first was a friend's extended family (13 people) and the second was a group of friends with people popping in and out all trip (11 max).
Group 2 had a "core" group and some others who joined us for a few plans but mostly did things on their own.
Group 1 insisted they wanted to do everything together. We proposed rope drop together, then family units branching off, with 3-4 TS meals together throughout the week. Family insisted on basically ALL TS meals together. (We sent emails out about seven months ahead of the trip saying "We will be at Epcot, choose from a, b, it c - and if they chose differently, someone changed until they were all on the same page.)
My friend and I divided and conquored to get the ADRs we wanted.
About three months before the trip, we sent out an email requesting "must do"s. We only got a handful back (which is fine.) From there, we built our touring plan. We normally do RD - nap - PM park, so kept with that schedule. Family was given it to review but did not request any changes.
FPs were made at 60 days - divide and conquer with me, friend and TA.
As we approached the trip, with an eye on trying to keep 13 people on the same schedule, we laid a few ground rules. These boiled down to:
1) This is the "core" plan the 'tour guides' will be using. You can join or not - we are fine either way - but the plan will not change based on one group member's whim. If you want to do something else, go do it - but the core plan will not change.
2) Every morning had a "bus time." Expectation was those participating in RD would be at the stop by bus time and we would board the first bus after that time. Those running late were welcome to join later, but we WOULD NOT wait at RD for latecomers. Please join at second ride.
3) Blowing off ADRs results in financial penalty. (We explained $10 fee etc.)
We used a "daily sheet," every room got one sheet per day stapled in a packet. I felt like they were a little drill seargent-y but they LOVED them. Both trips. We still had lots of flexibility (more than it looks like) but it gave clear expectations per day. Example here:
https://docs.google.com/document/d/1KPn5MhMuNlgdH1OHf4CxEEi4__PPt6D_4YWAlDBvrOI
Early in the week, everyone joined; we wore them out as the week went on but the clear-cut expectations helped a lot. There was no drama about who wanted to do what, everyone communicated and we never really got "derailed." Family's family of course

but all in all they had a really good trip.
HTH!