To create a new customers' group, you need to go to the PrestaShop Backoffice.
Here are the steps to follow:
From PrestaShop Backoffice > Customers > Groups > Create a new group entering all necessary information
From PrestaShop Backoffice > Customers > Create a new customer associating it to the group you have just added
You can now associate this new group to categories and/or stores
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