That's a tough question to answer, but 300 mostly return visitors is a good start. What percentage participate? I'd say in some ways you are ahead of the game, but you need to take some steps to ignite growth:
1) Ensure you are both engaging users more and creating more content by stimulating discussion. Start topics, bump old ones, etc. Be provocative.
2) Make social sharing easy so your users can share your content on Twitter, Facebook, etc. Spend a little time yourself ensuring that new topics, etc. make it into those channels.
3) Optimize the site architecture for search. Free traffic is your friend.
4) Spend some time and/or money promoting the site on related sites, via press releases, etc. Do some link dev work to get visibility on relevant sites - this will drive direct traffic as well a help with search traffic.
With a stream of regulars, you should be able to crank things up easily - many startup communities have to resort to begging friends to post or creating bogus users, neither of which is a great strategy.