Making a new site (with 10 pages of notes on google docs), and the builders seem to be too costly. for 10-20% equity. You will have to know how to make a social network/membership based site.
Assuming you're a business guy, I'd suggest you raise some money or validate your market first rather than waste your and other people's time with the age old build-my-idea-for-me-for-free-and-i-promise-you-fame-and-glory crap.
One very simple way to do that is to get a cheap wordpress template or whatever and get people to sign up for beta access keys or a newsletter.