The Cabal

About halfway through the trip you leave the compartment to go to the toilet at the far end of the train. You open the door, stride over to the urinal and unzip before something highly unusual catches your eye...

The girl is gagged and bound. You're not sure what to do. Do you untie her? Did she do this to herself? Maybe she's some kind of exhibitionist.
In the end you decide to just quickly finish peeing and leave. Maybe you shouldn't say anything. The angst you're feeling must have shown on your face, because when you get back to your compartment Hannah asks what's wrong.

"Eh. I don't know how to say this." You hesitate. "Back there, in the toilets. There was this girl, tied up, gagged and naked. It was weird. I wasn't sure what to do."
"It must be The Cabal." Ernie says.
"What's The Cabal." You ask.
"The Cabal is a secret group within Hogwarts. Highly exclusive, highly secretive. Some people don't believe they exist, but they've done enough stuff that most people believe in their existence. Nobody knows who is in the club, where they meet, or what they do, all we know is that they are mostly a sex club. It's more than that though, The Cabal rules the school in the shadows. There are rumours that their members get extra perks in addition to wild, lavish parties." Ernie explains.
"What sort of perks?" You ask, intrigued.
"Oh, all sorts of things." Hannah says. "Their members supposedly do better on tests, are able to borrow unique books from the library, have access to all sorts of interesting potions and spells, occasionally get exotic food. The lavish sex parties are the main reason everyone wants to join though. But the only way to become a member is to get invited."

"It seems kind of shady. Why hasn't it been shut down?" You ask.
"Weren't you listening?" Ernie exclaims. "Nobody knows who's in it or where they meet, how would you shut it down? Also some people suspect that the group has ties to the staff, maybe the staff are receiving kickbacks from them or something. Anyway, just know that if you see something like that bound girl again, it's probably The Cabal. And if you ever get a mysterious envelope that's purple with a cream border, open that thing immediately, because it's most likely an invitation to The Cabal."

The conversation with Ernie and Hannah has given you a lot to think about. It would be cool to join this Cabal but you haven't the faintest idea how to go about it. Oh well, you've always been resourceful, it's something you can work on. For now, you can feel your heart rate increase as you watch the lights of Hogwarts become larger. Your year at Hogwarts is about to begin.