If you're at ground-level and just starting out, you can buy your way into a few (as jmduke pointed one out). I highly recommend joining Rob Walling's (rwalling) Micropreneur Academy and branching out from there. The cover charge is a bit pricey at ~$50 a month, but if you contribute to discussions and look for ways to help other people (this is key), you'll find your way forward into some other great communities.

Another alternative is Amy Hoy's (ahoyhere) 30x500: http://30x500.com/

Those are a couple of the public-access gates. Enter therein, and bring with you optimism, a willingness to share knowledge, and determination.