The number one thing I would suggest is to make sure your forum is ready for members. The #1 thing that stands out about your example is trying to view it on mobile. I would suggest optimizing your site for the mobile screen, several of my members only post from their phone so I’ve done everything I can to make that as painless as possible.
Once your sure your site is perfect I’d ask your friends to post, also check out the other SEO forums and try to help out members on those sites. You can link to your site in your signature, it’s a slow process but proven to work time and time again.