How will this site attract new members? It takes time for Google to like you. It comes down roughly to (a) site structure, load speed and other software issues, (b) how many other sites link to this site along with how much Google likes those sites, and (c) high quality changing content that's relevant to people's searches. There are many other aspects but I haven't been paying attention to SEO for the past couple of years.
PF is at the top because of its age, reliability, crawlability, and masses of high quality content. Its rise to the top of the search results is one of Paul's great successes. If we can get as much high quality philosophical discussion going as possible, as soon as possible, we'll have some chance of picking up people from Google. But the competition is immense and it won't happen any time soon.
The Google bot has visited the site, I think, but it didn't stay for long. It may be back in the next couple of weeks, and hopefully then it'll take a closer look and index more pages.
There may be some technical barriers here. It's possible that this forum software is not very SEO-friendly, and because I'm not in control of it I can't quickly make the necessary changes. However, technical issues, i.e., aspect (a), are the least important when it comes to pleasing Google; if there's relevant content here the bot will find it.
One thing I can do is set up a Google Analytics account and add the site to Google Webmaster Tools. They'll help me see how the site is performing month to month.