You must have been a member for at least two months (60 days to be more exact) and have over 256 posts. Request to join and you should be allowed in once a moderator accepts your request.
Post things you don't want on the main page here. If you don't want it to be found by Google, post it here.
You can join private forums through your user control panel (click the "usergroups" tab and apply for a group).
Please do not share what's discussed in private forums to anyone not in them. There's a reason they're private.