If you want to attract members to your WordPress membership site, the best way is to use free content. No matter what niche and type of site you have, you can’t attact members without free content. What is even better is that your free content goes viral.
Good quality content makes a WordPress blog successful as it will rank well in search engines. It helps people to notice you and bring them to your site where they can be one of the paid subscribers one day. However, you cannot make it happen if your content is not popular online. One way to make your content popular is by turning a content into viral content.
Read how its done here: http://skyje.com/wordpress-membership-site/