Lots of people who have a WordPress blog want to charge for their content, so they need to be to restrict membership. I am currently developing a WordPress site for a client who needs precisely that type of feature. There are actually very few plug-ins on the market who can do a satisfactory job of it, and most of them cost money. Well, except for one.
Memberwing.com has a wordpress membership plugin that is free, if you are willing to part with your email address. It offers four levels of membership, with SEO potential in allowing snippets of posts to be indexed by Google.
Enjoy.