First you should probabily come up with some items that you think would interest a lot of people.
Then you would probabily try and get lots of people to come and post in this thread(or a new one around the specific item) so that there is an idea how many people would be participating in the group buy.
Then you would want to contact vendors and ask them about what they can do for pricing for items being delivered to tdifest for a group of your size.