What great fun!
I'm an old geezer now, but have been on many a group trip to themeparks. One thing I would strongly suggest is that you make some sort of plan in advance - especially around meals and rides that not everyone wants to do. I can't tell you how much time I've wasted because two people in the group (let's call them Bo and Jo) wanted to ride something and the other two (Lu and Su) didn't so they said they'd wait. Then, about 15 minutes into the wait, Lu decided she was thirsty and headed off to find a drink. Meanwhile, the ride ended, Bo and Jo returned, but Jo had to potty, so off she went. Lu returns with a Dole Whip - couldn't find a drink and this looked better anyway, and Bo just HAD to have one too. Off goes Bo, and Jo comes out all refreshed, but wants to duck into the gift shop to get the Tshirt she had been debating. And just when Bo returns with HIS Dole Whip, Jo decides she's hungry too but for real food - not ice cream. You can guess the rest....Through all of this, poor old Su - that's me - is really just standing there directing traffic.
Do everything you can to keep the group together or else you'll spend most of your time looking for people and waiting for people.
Also, I agree with other posters. Try to get the other folks to tell you their must-do things, and be sure those get done. They will want to do everything but they probably can't.
And finally, try to act a little bit like you are discovering stuff along with them. Fortunately that's easy to do at Disney!
Your friends are very lucky. I think it is great to have someone along who knows where to go and what to do.