The code was never fully finished (as most ambitious projects never are in a volunteer setting), so it can look a bit confusing from the outside.
https://github.com/bbqsrc/mutiny-forms is the repo for the join form. It was meant to hold all kinds of form-related stuff, but in the end it only ended up being the join form.
I’m Brendan on our IRC, so if you have any questions and want to hit me for having shit code, join #ppau-pwg (Programming Team channel) and I can walk you through the entire mess.