There is a 100 lb minimum order from the putty manufacturer (a little company called Dow Corning), so a group of people will get together and order varying amounts of putty, so that total order is at least 100 lbs, and a multiple of 50 pounds. Bulk putty comes in exactly two flavors: Coral (the traditional putty color, about the same as the background color of this page) and white. Coral putty is nice and all, but you cannot alter it very well, except to darker colors. White putty is perfect for mixing with colored powders or glow powders.
For a pool to occur someone must volunteer to organize the whole works. Tracking orders, gathering the funds, ordering the putty, receiving the bulk order, cutting it up into individual orders, and mailing it out to the participants of the pool. This is not an easy job, as you can imagine.
Perhaps someone who has run a pool could comment more on the process.