Wow, 12! I'm not sure what is the best, most economical way to get from MCO to WDW and back with that many people.
If I were doing this, I'd do some research on either Mears shuttle, or a Town Car service and then compare those two to renting a big van or 2 rentals.
Regarding rental companies, I've only heard bad things about L & M. I always use National and I belong to their Emerald Club so I can bypass the counter and pick my car from the appropriate aisle.
I also have heard good things about Dollar and
Alamo (sister company to National).
If you do want to rent a car, then I suggest you check out
www.mousesavers.com under WDW Transportation as she has LOTS of info on discount coupons and codes to get the best possible price for the rental cars.
If you want to try a Town car service, I've heard only good things about
www.tiffanytowncar.com.
Good luck!