I used to be a member of that. Allow me to talk about it for those who don't know.
You write in and say you're attending, and they put you on a guest list. It's required for entry. Then the event comes, and you pay the fee to get in. Also required. Cash only, as I recall. Beer and water are free, and there's plenty of both. Go in the room, check your clothes - don't remember if there's a fee - and go make friends.
Be advised: the average age of the group is 53. Not a bad thing in any way, but if mature gentlemen are not your thing, you may be disappointed. And oftentimes, at least in MY experience, the party can run dry and turn into a bareback affair. I also don't recommend kneeling in the large pool they put out - it didn't have any padding under it when I was inside, so it'll hurt your knees to be down there for catching piss. Solid concrete. Problematic. And don't wear your comfy new tennis shoes with arch support past the clothing check! The floor has no drains, and your shoes will be ruined with the odor of piss. Some may like that, but forget about ever wearing them in public again.
It's a fun event, and you should go at least once.