Optimize press has themes for the membership site but you will also need something like s2member (which you can get for free) to secure the site. This would cost you around $97 optimize press but it also allows you to create sales pages and squeeze pages and also a membership site.

Hope this helps!

Best Regards,

David

I think most of the themes will work fine you need a good membership site plugin , those are the best in Mamadou's opinion Paid Memberships Pro and Restrict Content Pro.

Before Nanacast, I used to look for WP themes that had a primary and secondary navigation bar and 1 or 2 sidebars. Most StudioPress themes meet this requirement.

Here are some good, free StudioPress themes that can get you started. Free Genesis Child Themes From StudioPress - WPSpeak.com