Everyone will tell you that your forum needs quality backlinks and good content, but beside these factors, you also need to care about more SEO techniques as increasing social networks signals, building internal links or standard on-page SEO...they are also helping SEO for your forum.
I agree with everything that @David Beroffsaid. And speed is important as well. Try to optimize your forum to make it load really fast. You may use a content delivery network (CDN) if you like. Oh, and try to make your website more secure, since you have login. Using SSL certificates is a great way to improve your site security.
Now the classic SEO optimization is becoming less and less important, since Google’s robots are improving very quickly.
To optimize and improve attendance, it is now better to add to the site demanded content, game FAQ, convenient rating with sorting. It is also important to do contests with prizes.