Well...the Secret Spell Casting Club Facebook Group IS obviously a secret. So we can't tell you exactly HOW to join it here on a public page.
Once you become a Spell Casting Club Student, simply sign in to your student area and click the "Instructions/Welcome" link on the top right of your student area.
Scroll down this window until you see "SECRET FACEBOOK GROUP".
That section contains simple instructions for joining our SECRET Spell Casting Club Facebook Group.