How to Join Private Forums
Go to your User CP and click Group Memberships.
From there you can pick the groups you would like to apply for.
Each group sets it's own rules; acceptance is not guaranteed.
I can't find the "Guide Developers" forum, although my request to join was approved.
To make it easier for people to join Private Forums, change the title of the Private Forums from:
"Private Forums - Go to "User CP" and then "Group Memberships" to join these private forums."
"Private Forums - Click here(link to Group Members) to join."