The basic recipe is that someone finds a venue and some workers, then does some publicity. Men turn up expecting to have sex with as many of the workers as they can manage in the time allowed having paid a fixed fee for admission. Depending on the venue and the format, you can be all in one room or spread out over several, but typically you will have at least of couple of men around you throughout, either being sexual or watching.
They tend not to be hours of non-stop shagging - you should be allowed breaks - but you do need to confirm what the 'ratio' is (how many men per woman) to see how busy you could be, as well as how many men they are hoping for (not everyone who expresses interest turns up...) You can then do the maths as to how many men you'll be expected to be sexual with and how much per man you'll end up with. So in a 'two men per woman' party with six attendees, you'll probably have at least four of them over two hours. If the ratio is more than that or length is longer, the number will be higher.
Whatever the result of that sum, you want the money up front and certainly before you start being sexual with anyone. If fewer men turn up than the organiser expects, that should be their problem, basically.
As with any other form of work, there are people who love working them and those who wouldn't touch one. The working conditions can be nicer than many parlours, there shouldn't be any sitting around waiting for clients, and if you like being watched while having lots of clients in a day without having to pretend to be interested in them as people, it's perfect. On the other hand, not everyone does! You can also have clients insisting on getting their 'money's worth' being determined to spend as long as they can inside every woman there having popped erection pills first, which can cause resentment among both workers and other clients leading to an unpleasant atmosphere.
The other downsides are the two kinds of negative attention they can attract. At one point, parties in London were being targeted by criminals who reckoned that a dozen or so men with no clothes on paying one or two hundred pounds cash each made a very easy target. It didn't help that the other unwanted attention was from the police. As has been said, the event makes the venue a brothel - 'brothels on wheels' was the memorable description by one police officer - and some police forces don't like them. You wouldn't be committing an offence, but you may find all the money goes as evidence in the event of a raid.