In order to join, you must send The Leader, Danny, a passport-sized photograph, along with a note to say who are, and maybe some little personal information too. This small cost (of both money and effort) proves that you are not someone who says they'll do something, but rather someone who will actually do something.
The photograph must be sent by post - no e-mail submissions are allowed (partly because it kind of defies the point, partly because Join Me is about trusting a stranger enough on a whim to get involved, and partly to save Danny's inbox).
The photographs themselves are currently kept in Danny's flat, with the exception of the original 1000 - they are buried in a field in Switzerland as a tribute to Danny's Great Uncle Gallus.
Not every Joinee was happy with this idea - Joinee Benjamin, for one, refused to send a proper photograph of himself (he used someone else's photo) as he believed Join Me to be a sinister cult like The Finders. It's not.