I would choose only one destination either WDW or Universal, but when choosing you need to be smart about it, and see what you can get for money and how it fits into your budget. Transportation at WDW you can use ME ( Magical Express Bus service)to get to and from the airport and resort and back again. I would suggest looking at a family suite as there are 2 bathrooms which is a must for larger families or larger groups in your case I think that they sleep 5 or 6. These rooms have a small kitchenette, a small frig and microwave. You might want to try to come when free dinning is being offered, even if not you might want to include this, in the cost when budgeting. I as a parent I would want to make sure my kids meals are included, also not everyone has the same ability to pay, so one price for room, park tickets and dinning plan might be the way to go. As a team mom for 4 years, adding in meals was always important, again I can tell you that I have had to pay for a kids meals as their parents forgot to send any cash for meals. This way if something happens lost wallet or whatever, at least meals are included.
I do have a thought would it be possible to do a caravan from where you live, driving would cut cost and you could make it part of the adventure. Also you could bring water, snacks which helps cut down on the cost. I used to drive for my daughters team, you might only need 3 large vehicles to get everyone in. Add in cost for gas to the budget.
If flying is going to be the way to go, see if anyone has AAA to help, we used then for a trip and I surprise at the cost was pretty low, around 300.00 for Round trip, also start talking to mom and dad about airline miles to use to help with the cost.
From now till you go ask for Disney gift card, for birthdays, Christmas to help add to your spending money.
Also just so you know that when talking about a trip like this is fun, but when it comes down to it some might not be able to go or bail out at the last minute so you need to be prepared. If someone does how will this affect your cost. Lots to consider and plan for.
You need to create a real budget, and have a meeting not only with friends going, but with mom and dad included... So that everyone is on the same page and understand all the cost of the trip and what it includes, sleeping arrangements, any medical issue. The more you are prepared and able to answer questions the better it will go.