I have organized a couple and been to several more. They are a pain to organize. The best bet is a club. Phone ahead and see which nights are best for unaccompanied males. The setting is safe and there is no need to recruit. The club might have a gang bang night where entry requirements for males is relaxed.
If you can't get to a club you have to start with advertising. Dating sites can offer GB sub boards. AFF has some by locality. Craigs List was the the place to advertise, but no longer. If you can set up an ad state the date, time and place such as a hotel. Insist that the players provide voice phone numbers and are available a few hours before. Speak to each one, give him the room number and any time window. One event required the players to be in place so the lady could make an entrance rather than wait all evening on the bed. Hotel rooms can get crowded and players might be expected to bring their own towels and safe sex supplies.
A lady interested in a GB needs at least one gentleman to control things, to watch her back. Two men isn't bad. When she says no she has friends to reinforce the decision.