Tell your friends and family, put it on social networking (Facebook, Twitter, LinkedIn, etc), try to advertise it locally with fliers and stuff, buy some Google AdWords credit (and equivalents for Yahoo and Bing if you want), make sure your SEO is in good condition.
About the membership thing. What I generally see happen with startups is that it is free initially, but they make it very clear that on X date it will no longer be free, but that legacy accounts will have some sort of discount or benefits. This encourages users to try it out while it is still new and young, and also encourages them to stick around because they think they are getting a good deal.