Sondi, There is a lot involved.
I personally average 40 - 60 agents at my brokers open houses. It is a matter of networking and whom to contact.
A FSBO doing a brokers open? You are going to end up with cards and information about Realtors and their companies trying to get your listing. You may not get any agents. Quite a few realtors do not show FSBO or Flat Fee company listings regardless of the "split" you offer. As far as going about it, due diligence, like anything wlse, get out the phone book and get fax numbers of the local offices, make a flyer and fax it out to the offices for the day you want to have the Briokers open.