There's been a lot of promotion on here for ADCO, Troy, Magpul, and others. I don't know how the logistics would go for a "group buy" but it seems that some sort of coupon code for a group discount may be easier.
Those that have been doing this promotion, as has been said, usually already have these items. But if it's a store-wide group coupon code, maybe there could be other things they offer that could be purchased with the discount.
That being said, if a group buy is the way we go, I say a discount on a complete set (front & rear) of full sized folding sights.