|There are lots of systems that allow you to use other systems (such as facebook or twitter) to authenticate so that they can post a comment. One option is to have a users table in your database where you store all your users and then a column in the table to store what type of user it is, i.e. facebook, twitter, google, your own member, etc.
When they want to login you'll need a separate link or a dropdown for them to select how they want to authenticate (facebook, etc).
Based on what you describe here you really just need to support multiple types of users by adding a flag to your user table and then also by providing the various login options.
There are only 10 types of people in the world, those who understand binary and those who don't.