I've been buying new cars for 20+ years (whoa, that's longer than a few here have been on here planet Earth!), and this whole private-purchase "group buy" dealio is new-to-me. With that said, I too was VERY skeptical as well. I believe it'll comedown to the dealership business-model (high-volume, large market, allocation limitations, etc) AND fleet-sales manager interest & inclincation.
I think I've decided that I'll do an out of state/region purchase via one of these group buy if the discount is 10% off MSRP. Otherwise, I believe there can be at least $1,500 or so (+/- $500) through a car-buying service, which is about 5%'ish discount.