My wife and I have attended a few invitation-only events at lifestyle/swing clubs over the years. Two were gang-bang/group sex themed events, one was a halloween themed event.
Our motivation for the gang-bang themed parties was to experience a logistically lower "overhead" bang, that we didn't have to do all the leg work to put together. The club in this case provided the men, and women, the venue, and the monitoring/screening. The downside is that my wife enjoys bareback fucking with screened, healthy partners, and the club's rules for these kinds of parties was mandatory condoms. Both parties involved women who wanted to be "centers," or "bangees" - the clubs have different lingo for women who want to be the center of attention and focus of a gang-bang. They have a person or two that acts like a monitor or sort of floor manager for the event, keeps things going and makes sure everyone follows the rules; condoms, paying attention to the center, stuff like making a guy stop, or enforcing "no" if she's truly not interested in a particular guy's attention, and so on.
The two "down side" aspects of these events were the mandatory condom use, and that my wife didn't get to pick all the guys. The latter means that she's in a position of either saying "no" to a guy she doesn't find attractive, or that has a small cock, or allowing him to participate. The club did a good job of screening the men, in that they all had good hygiene (clean), but they ranged in attractiveness and cock size - some of the men were overweight, hairy, uncut, or had smaller than average cocks.
Both events were kind of a mix of "dog-pile" and "running a train" bangs, which was pretty hot, so I got to see my wife in the middle of a group of five or six men at a time, fucking, and being fucked, as well as taking one strange guy after another, in a variety of positions, sometimes one, sometimes two to three at a time. Both events used three or four rooms of the club, one bang/orgy on the sort of main stage area, and two or three other simultaneous bangs in the other rooms. There are so many men that there are sometimes two to three centers in each room, or some women "helpers" that act as fluffers to keep the guys hard for the center. That upside is that you can sometimes go from room to room as a participant, if the club's rules allow it, or watch other hot women being fucked and banged, again if the rules allow it.
The halloween party was just basically lots of couples, and a few single women in costume getting together for sex, either same-room, no swap, full swing, or in a couple of cases sort of "mini-bang" group sex deals. One hot aspect of that event was a "dark room" - basically a completely dark room, with no lights at all. The idea is you go inside - the room had double doors, so that no light gets in the room at all, and just have sex and contact with whoever is in the room. Random, no-holds fucking, touching and feeling, without being able to see who you are having sex with. That was hot, but it can be disorienting and uncomfortable for some people. The feeling of dozens of naked, wet sexy bodies writhing around is pretty amazing.