Not completely sure, but from what I've heard it's just a regular gay party with a dj and a bar and a dance floor. Of course you've got the cabins so there're more opportunities to get sexy with other party goers, I suppose.
No working boys attend the parties, it is not a place to go and find working boys. The boys have told me this.
Sometimes an entrance to the sauna is included in the party ticket and the party goers will stay until the sauna reopens and the working boys arrive.
This happened one Sunday when I was there. There were some really hot guys that turned out not to be working boys, but regular gay guys that had attended the party and were staying. The working boys were polite about it but some of them told me they were a bit annoyed as these party goers would flirt and take up time from the working boys without any intention of hiring them.