Access user groups are used to give customers access to products and or categories based on their group membership.
The products/categories user groups form:

To use access user groups, you need to enable them and create at least one access group. A second option allows you to turn on or off the visibility of products and categories for the storefront for customer’s, that are not members of this group.
Please note: If the visibility for products and categories that have an access group assigned is turned on, all customers and visitors of the shop can view these products and categories and even add them to the cart.
However, a note will be displayed at the product details page, that the product is only available, if the customer is a member of a special group.
If the customer is not in the same group as the group specific product, he won’t be able to checkout as long as this product is in the cart.
After you created a access group, it is accessible through the user page at the admin area where you can assign access customer groups to your customers.
Use the “Add New Group” form to add a new access customer group to your groups.
Once you entered a group name and a group description on the “Add New Group” form, click on the “Save” button at the bottom of the form to add a new access customer group to your access group list.
The form also allows you to bulk change your groups. You can edit all details in the list and click on the “Save” button at the bottom to update your access group details.
To delete an access customer group, click on the “delete” icon on the right side of each item in the list.