We usually go with my parents, my sister, her DH and two kids, my older brother, his wife and daughter, and my younger brother, plus myself, my DH and DS. That's 13 of us so I know what you're going through!
The way we go about it is, someone decides on which park we're going to (usually me!) and I just say this is what we're (DH, DS and me) are doing, anyone want to join us? I want everyone to feel like they're welcome to join us, but also not obligated either.
When we head out in the morning, usually half of us end up going in the morning, the rest will text someone in the group when they want to join us.
I will make the ADR for all, and then update the numbers once I know for sure who is going. Sometimes it's not until the night before, sometimes it's months before.
Rooms I'm not much help on, my parents are DVC members so we end up with either a grandvilla or (2) 2 bedroom villas. But if you're all staying at the same resort, I would have everyone request a certain building or section. While it might not work, Disney will do what they can to accommodate the requests.