We did a group of 13 a year ago. We stayed onsite so I think it was a bit easier....In terms of people could come and go back to the resort at will. How many vehicles will there be?
One thing that helped us tremendously was that there was one person in charge (me!). So we didn't have the 'ummm...what should we do next - Space Mtn or Dumbo?' type discussions -that can be time consuming with so many people. I took control and mapped out our itinerary with occassional tweaks based on input (if everyone is hungry for lunch, etc.). That's another point - try to figure out where to eat upfront - that discussion can take forever. A few times I would say - OK - here are the choices - we could go to Cosmic ray's that has X, Y and Z choices or we can go to Pinnochio Village Haus that has A and B....And then we would split up for lunch and meet back in the middle. Now everyone I was with did not WANT control over these things - so it was perfect.
Be prepared - Even going across the park with that many people can take a long time. We had a picture-taker in the group and would frequently think we were all together but he had dropped back to snap a couple shots. Not a huge deal - but things take LONGER than when you're just with your family.
Make sure expectations in terms of togetherness are established (staying together 100%, splitting up into separate families at all? Also - if there are definite divisions in ages of kids - older kids might not want to do too much little kid stuff, etc.).
Group pictures also take a long time....Just in case you were considering PHotopass...At least for us by the time everyone was in one group and would stop talking - it took a long time, so we didn't do too many whole group shots.
Have one person in each family hold the tickets for their family. WHen time comes for a fastpass - have one fastpass runner grab the baggies with each family's passes.