First off any group buy needs to be though out very well. And a plan needs to be set. We have formed a few groups like this on salehoo and they failed miserably.
If your considering something like a swap buy where you buy a mass quantity of products. You will need to arrange the Purchase. The pickup. What party's will be there for the pickup , who gets what from this purchase, and how you plan on distributing the product to the members of the group buy. Shipping is going to be your biggest concern.