My group was "only" 12, but big enough to be a little help, I think. Twinnersmama had great advice. To add on:
Have a trip planning party (meeting, really, but party sounds more fun!) Decide whether you will have a designated planner (or several) or whether you are only going to loosely say you're all going during the same week and everyone is on their own.
State from the beginning that you will NOT expect to all travel en masse at all times.
Tell everyone that there CAN be a LOT of advance planning involved in a Disney trip and find out who is interested in being involved in it. You need to know who wants input and who wants to just show up and be told what's happening.
Tell everyone (or at least 1 adult per room reservation) to sign up for a My Disney Experience account.
I liked having 1 table service per day as our "anchor" where we'd all be together at least that one time of day. In my case, this went hand-in-hand with picking park days in advance - decided how many days to devote to each park; picked favorite restaurant choices in/near each park; used crowd calendars to help narrow down which day should be for each park; called WDW-DINE to set up 1 reservation per day
It's highly likely some people in your group will not like talking about where to eat 6 months in advance (reservations open 180 days out and especially with a group, are highly recommended). IF you're going with table service meals, make the reservation for all 20 and adjust down later if needed for people who don't want to participate. It is MUCH harder to add people later than to subtract.
Early on, encourage everyone to browse the Disney web site and watch the planning DVD. Tell them that FP+ exists and as that booking window gets closer, you'll have to decide how you want to handle that selection process (every man for himself? or all for one and one for all?)
For each day, I like the idea of saying MY family sub-group is going to be in line for the buses at X. If you'd like to join us - see you then! Otherwise, you know when our first Fastpass or dining is and we'll see you there.
If you make a majority of the plans, I liked making a paper backup of critical info (for myself just in case my phone battery died or wifi in the park was sketchy) and giving one to everybody. I made 1 wallet-size card for each day with hotel confirmation #; dining confirmation #; Fastpass times for the day; park hours and fireworks/parade times for reference. This way, I felt like even if we were separated AND phones failed for some reason, I was not responsible for anyone else having fun or knowing what was booked for them that day.
Here on the DIS you'll get lots of great advice, but whenever you mention a group trip, you'll also get many people suggesting DON'T DO IT. . . good to really consider the makeup of your group and whether this is a good idea - vacation time and money are precious and not everyone has the same notions of what constitutes a good time. But I had a great time with my group and hope you do too!!