WordPress.org

1. Upload user-cats-manager.php to the /wp-content/plugins/ directory
2. Activate the plugin through the 'Plugins' menu in WordPress
3. Access the Categories And Users in your settings options.
4. Select the user. And click on edit button.
5. A list of categories and subcategories are showed for you.
6. Select what categories that user can use write/edit posts.
7. Click on save button.

Note: Administrators can write on any categorie by default, so use editors for new users.