Step 1: Inviting Your Friends
The first thing to keep in mind when you're deciding who to invite is this: not everyone enjoys thrift store shopping (maybe you count yourself among them, but I rather hope not). Some people were raised to believe that thrift stores are dirty to the point that they aren't worth going into. Other people believe that these stores should only be visited by shoppers who can't afford merchandise in other stores.
While it is true that these places can be dirty (no store will be completely clean, and you should disinfect your purchases when you get home), there are deals and treasures in thrift stores that you can't find anywhere else. As for only being around for people who absolutely need them, so many thrift stores create jobs and donate money to their communities that I think they're worth going to.
Keep the cycle up by donating to and buying from the stores you visit. Look at it this way: you're donating and buying things for cheap rather than selling your old items and spending that money in more expensive stores later.
Once friends agree to go, choose a date and time that's best for everyone. Try to split people evenly into groups that balance out as far as shopping experience is concerned. Make sure that everyone distrubutes phone numbers, and if at all possible, print out a list of addresses for the stores so everyone knows where they plan on ending up.