The best way to join a shadowcast is to go to a couple of shows (most casts like to take on people who come regularly; it shows you're reliable). Introduce yourself to the cast afterwards; this will give them a chance to get to know you. Ask how to go about joining. Most casts have a vote or perhaps an application form; there may be auditions.
It's good to know what you'd like to do - is there a particular character who intrigues you? Do you want to do something like props or lighting, instead of getting onstage yourself? If you do want to do a particular character, start assembling a costume. This shows you're serious.