You can only join Pinterest through invitation only. If no one invites you, then you can go to their website http://pinterest.com and click on "Request an Invite". This will bring up window, asking for your email address. When you get an invite, you must register through Facebook Connect or Twitter. If you do not want to this, you can get started by going through either of these accounts initially, and then unlinking or deleting them once you have successfully joined Pinterest.