The only thing that will make the community grow in the long-term is generating more content, it's useless that people know of the site if when they get here they don't see anything of their interest; they'll go back to reddit because it's got more active users and a major flux of content. Sure, it's foolish to think a relatively new website will have an alternative for some of your most specific and niche subreddits, what matters is letting the new users know that even if that particular content isn't available for now, it can be available at some point, at to motivate them to create and manage that content on their own. The invitation system should stay as long as we get a stronger, more stablished community and more variety in regards to content.