It's a total mix, you truly have some genuine amateurs that you will find nowhere else and that were there only to drink and dance but ended up for some participating in the action one way or an other, to semi-pro that did this and maybe one or two other scene for other czech production like czechcasting for example but never really fully entered the porn industry and then full porn star.
If you want to laught, there a famous anectode about one episode featuring the daughter of the czech president of that time in one, she doesn't do anything, but is still in the room while all the action is going on. Made quite the headline back in the day.
Just know that the deeper in the past you go about party hardcore ( all the way up to 2003 I believe ) and generally the best amateurs actions you have, even though personally I still think Party Hardcore Gone Crazy ( the volume that came after the original Party Hardcore series ) still had some really good amateurs actions.
But since Covid and their new type of parties after the PHGC series, it looks more like Dancing Bear with only pro and semi pro and the rare occasional amateurs action than what Party Hardcore trully was.
Rise of social media, and other stuff, probably also made these type of thing pretty much impossible to make nowadays.