I do group orders for bird toy parts all of the time. I just put a few rules in place and if you followed the rules no problems if you didn't the next time you were not in on the order. I have been running it now for going on 4 years without any problems.
I made anyone ordering pay up front by paypal, they covered the shipping charges split among the items ordered. I just increased the price to cover it. (I don't pay shipping since I do all the sorting and repackaging and mailing out).