I would google "promotional forums" and join a few if you haven't yet. The ones I'm active on are Forum Promotion and Admin-Junkies. Do post exchanges with other members and once you get enough forum credits you can purchase posting packages. (I'm on the posting team on both forums btw.) The more you participate on other members' forums, the more people you can get to join yours. Also don't forget to promote on other places too like social media, Youtube and look into the best SEO practices to get your forum out there.