I just tried signing up for the site. My first hurdle was finding the sign up button. It's nearly impossible.
After finding the "New user" link and clicking on it, I'm presented with your community guidelines, which I understand make MeFi what it is. But, that page could still do with a sign up button, instead of the link buried at the bottom.
Anyway, I clicked on the "Go ahead and sign up for an account here" link, filled in my details and then I was presented with the "pay $5 to complete your signup" message. I didn't know it was going to cost me $5! I went back to the guidelines page, and I noticed you did mention the $5 there, but I didn't read it.
I expect this is the most common user experience of new visitors to your site interested in joining.
I don't know if improving these things will move the needle at all for you, but there seem to be a few simple things you could try to increase signups.