Steve, One way to do a group buy is to limit the buy to kits, blanks, bushings and perhaps extra tubes. If you don't people will drive you crazy with all sorts of oddball requests and you will be chasing your tail keeping up with it all.
It is also easy to import the kit and blank numbers into a spreadsheet and keep track of who is buying what.
100 kits is the level for the 25% discount so I usually say up front that the cutoff point to the buy is 100 kits or thereabouts. If you don't, you will have stragglers posting for days or weeks with 1 of this and 3 of that, a blank here and there or 4 spare tubes and it'll drive you nuts.
After several buys I found it easier for me to pay for the entire order up front then when everything arrives I sort out the orders, package and weigh each box to find the proper postage then email the people their portions so they can send me a paypal then mail the packages off.