I'm a big fan of buyers groups, it does give you more purchasing power, however it does have negatives aspects that you always need to be on top of before you set the wheels in motion as a group.
My number one tip when forming a co-op, keep it small!
That often means that you need to invest more as an individual, but it gives you a chance to make a decent margin throughout the group.
Number 2, keep the free loaders to a minimum. Take on members that have something to offer the group as individuals. By keeping it small and using more of your own money as investors, you don't need to take on investors only interested in seeing a return on their money without doing any of the work involved!
Those two tips will take you a long way to creating a successful buyers group, of course there is still much more that needs to be worked out within the group, but that can be done when you have the group of like minded people all together.
Top 3, all hook up through an IM so you can all share in the meetings. It's important that you are all on the exact same page so you can ensure all the wheels are moving in the right direction!
Good luck with it!