It sounds like you should be looking into WordPress multisite. Multisite is a way to run multiple sites on a single installation of WordPress. WordPress.com is an example. One big advantage of multisite is that it runs off a single set of plugins and themes. That means you can allow separate users/authors to only have access to certain sites while still only having to keep one site updated.
Also, with multisite, you’re going to have an easier time sharing blog posts. Here’s one tutorial. There are also some multisite specific plugins that offer similar functionality.
Finally, I should add that at the beginning of your post, it did sound like categorizing was your best solution. If you really want the separate user permissions, multisite may be the way to go, but I’d encourage you to read “Don’t Use WordPress Multisite” for a great list of reasons to NOT use multisite. There’s nothing wrong with using it, but you want to make sure your choice to do so is an informed decision.