You have to flood the forums, post a recruitment comment, bump it a few times a day for a week, then rinse repeat. If you see ppl looking for a clan, message them direct, or send them an invite. You must remember, there are thousands of clans to join. You can also invite through the game itself, so press R3 to view a guardian, if they are clanless, invite them to yours. You'll need patience too, but it will pay off.