Multiple blogs, one user list?

I guess the topic describes it quite well: our newspaper has 7 blogs running the WP system, and each one has its own user database, as would be expected. However, for obvious reasons, our readers would probably not want to register individually for each one, and therefore would mean that most people wouldn’t take the time, and would in turn not post comments. So I am wondering, would there be an enormous amount of recoding to do to make this a reality, or is there a centralized function/definition/variable that specifies the user table, which I could edit and be done with this?