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Since articles by default inherit the view permissions from their categories, here's how to set view permissions on a category.
- Create the category (using the +New Category button)
- After the category is created, click the Edit pencil icon for that category (upper right of the page)
- Click on the Permissions tab
- Use the check boxes etc to set permissions
Here's a little screencast of the process.
If you want to remove a Group from the permissions list, check the box next to the Group name and then click the Trash icon at the top of the Group list