To hide a blog post, brand, buying guide, information page or product category from a customer (or price) group, go to the Content menu in the control panel and select the content type. In this example we will use a product category.
Go to Content > Product Category. Click on the category you want to edit or click on the Add New button if you want to create a new caregory.
Scroll down to the Naming section and enable the box next to Require Login. This will restrict the category to only logged in customers.
The visibility per customer group (wholesale / retail / levels of customer, etc) is set on the product level on the product page. When you add a product you can indicate which customer groups the product is visible to and which category you want it to appear in. You will find this option in the Visable to User Groups section.
To view more information on adding category to a product check out How do I add a product category using the web interface?