I'm very annoyed. I managed to get an invite. I tried clicking it from my iPhone. It then said that they don't support mobile browsers and disabled the link for me to try it again from a normal browser.

Update: I went back and tried the link, and it seems like it actually worked. Or got my account into a half-setup phase where I could complete the activation. Interesting. I don't see any mechanism to send out additional invites, though.