Group buying is all about getting getting volume discounts, the more money the group has to purchase with will usually translate into a lower cost per item. Wholesale suppliers are what you would be looking for, and I am counting genuine liquidators in that as well.
How does it work, there really isn't any set structure. It essentially comes down to an agreement within the group.
Number one - Keep the group as small as possible to ensure larger profits
Number two - Open communication so each member understands their task
Number three - Get your division of labour correct so the load of operating the group effectively is spread as evenly as possible.
Number four - Consider the end game, how you are selling the products. Some of these types of groups tend to get together to purchase, but then fracture and go their own ways to sell after they all take their share of products.
What can happen in this case is that everyone ends up trying to sell the same products on a site like eBay, of course competing against each other in the end and making very little or nothing at all for the effort.
In my opinion, when you create a buying group, develop an end game that will take you right through the sales process as a group as well. Make a group profit, then look at dividing profits or rolling them back into a larger purchase!