I've not done one but it seems somewhat straightforward if not a fair bit of work. You have to find your supplier, whether that is a lhbs that can cut you a deal, a brewery that will pad an extra pallet to their next order, or a wholesaler. I believe a pallet is your minimum purchase to get any substantial cost savings. After you get your specs; price per sack of which grains, number of sacks per pallet, etc you advertise it to your local audience. You will need a place to do the distribution. Will it all be full sacks or will you allow splitting of sacks on site? You will need a large accurate scale if so. The purchase will likely be going on your credit card and then you collect from everybody by PayPal beforehand or cash/check at delivery. It's your group buy. You make the rules. Just look at some of the other group buy threads and go from there.