In case anyone does not completely understand, registration has been closed off to new users without a invite/registration code. Any new member that wants to register will have to be invited by an existing member of this forum. To invite a new user you have to generate an invite code first which can be done by pressing "Invites" and then clicking "Generate Invitation Key". You would then give this key to your friend that you want to let join through a SECURE MEDIUM.
You do not text it to your friend, you do not post or PM it on reddit, you do not post it on pastebin.
You send it through your friend utilizing GPG or through something like XMPP with OTR enabled.
Once that it is done, your friend should now be able to register here at the forums by filling out the normal registration information and by copying and pasting the invite key into its appropriate box on the registration form.
Finally, be careful who you send these invites to, especially if this a permanent measure and not just a means to control the inevitable publicity rush. Giving someone your invitation code is like saying you vouch for that person and that you are willing to put your reputation on the line for them. Do not give one to someone you can't trust or be held accountable for.