We are going in a few months and there will be 21 of us! So, I definitely understand your concerns and questions in getting everything organized for a smooth trip.
We had to call the Disney Dining phone number to book our ADR's and for all of them we had to book two separate reservations to accomodate the group. For example, one reservation for 10 at 6:20 and another one for 11 at 6:30. It seemed very difficult to get us to sit all together, but you may have an easier experience with this as we did not book exactly 180 days prior.
We have not yet made our Fast Pass selections but it looks like we are able to click on the people who would like to be included.
We have had a few "meetings" which were very helpful as well as various emails asking if people wanted to be included in ADR's or other activities. However, I do think it is critical that one or two people take the reigns and make decisions for others IF that's acceptable with your group. Of course, if your group has strong opinions or have must do's, then including them more will be important. My sister in law and I are the only ones who have been to WDW before and we made the decision to schedule which parks to visit on which days, and will ultimately pick the Fast Passes.
Another tip is to make a table or spread sheet that can be sent via email or Google Drive and that includes the day by day schedule of events, hotel info, rental car info, etc. When we have one of our meetings, everyone gets an updated copy. I am hoping that with this document people have an idea of what is to be expected instead of me feeling like the perpetual
travel agent who is constantly being asked, "what are we doing?", "where are we going?" , "what time do we eat?".
Our group will also go on a cruise after WDW where another 13 people will be joining us!!!