Your comments

So have it in topics and people only need to join the topic if they know they are at risk of being turned

Mabye like an avatar thing? Like Mii's or something, you can pick a face, a body type, hairstyle, so on?

A site nominates a player and they all come together. The first challenge could be an RPG contest, the second, write a backstory, the third could be like a quickfire quidditch match, where questions about HP are asked. The winner could receive a month of VIP or something like that...?