Convert an existing website to a members subscription Digg.com clone

Required to convert an existing website to a member subscription [login to view URL] clone website. There are two types of users 1) free user that can browse the content of site at no charge & 2) the registered user(member)that can post content on the site. This user is allowed to choose between different types of monthly plans available at registration. Each plan allows a certain number of transactions/posting per month. If user reaches the limit of transcations for that plan, he is allowed to upgrade to the next by paying the difference.