I haven't participated in a GB, however I have thought about it and would like to possibly try it one day with a couple I knew AND the other guys involved weren't just invited for sloppy seconds from off the street but guys that the couple respected and knew to some degree.....I think the lady should have a small group that she has had experience with at one time or another and organize a hot get together.
As mentioned in a previous posting, the thoughts of sloppy seconds do turn me on...however, I would think of it not as one after the other just penetrating her but perhaps one guy receiving oral while one is inside her and another maybe getting a "helping hand"...then maybe a few might switch or one might "come off the bench" and penetrate her...LOL
And yes, I would have sex with a GB professional...