A buy group, also know as collective buying, is a group of noncompeting retailers that analyze, select, and purchase products and services together, such as advertising or equipment. The buying group offers products and services at significantly reduced prices on the condition that a minimum number of buyers would make the purchase. Generally, websites will feature a "deal of the day", with the deal kicking in when a set number of people agree to buy the product or service. Buyers then print off a voucher to claim their discount at the retailer.
Source: www.subicom.com