I've been enjoying group sex since I was 18; I'd define a sex party as more than four, myself included.
These are great provided there's a proper balance of tops, versatiles and bottoms (which isn't always the case, at least here in FtL). The ones I've participated in over the last ~six years or so have been, generally, much more laid back, with relatively brief (10-15 mins) periods of activity in twos and/or threes before changing up, rotating, taking breaks, chatting, etc. In a lot of ways, they're just a lot of nude guys hanging out with sex going on here and there intermittently.
There's almost always at least one guy on the computer inviting more guys in, as guys will come and go over the period of the party, which can last anything from an all-nighter to an entire weekend. I've always been surprised by the really high caliber of men I meet at these things, at least as far as looks and sexual agility are concerned. Frequently couples will arrive together and play both apart and with others. There's usually porn playing on a TV with the volume turned off and music playing.
A really big caveat: almost all of the ones I've been to recently have been PnP affairs (PnP=Party & Play, where the "party" is generally meth). I don't party myself, but as a non-versatile top that definitely works to my advantage (no hardon issues), and I'm usually pretty cool about what others do as long as it's understood that I won't partake myself. Because these tend to be PnP, you can generally tell who brought the drugs because he's inevitably the least attractive guy in the room
The other thing is that, for a variety of reasons (including drug use or serosorting) safer sex precautions have rarely if ever been taken into consideration at the parties I've been to. OTOH, I've also met plenty of guys who set and keep their limits, including just wanting to watch; as in real life, not everyone's interested in anal sex anyway. In fact, the sex is generally pretty vanilla unless they're specifically themed as "dungeon parties", in which case one should know what one's getting into by attending such things.
The last thing I'd caution against is hosting a sex party oneself. It's much more fun (and poses a lot less risk) being a guest.