Salesforce B2C Commerce includes a set of standard customer groups (Everyone, Registered customers, and Unregistered customers).
You can define additional customer groups and assign customers to them. For example, if you create and activate a promotion that is targeted to registered customers, only a registered customer would actually see the promotion on the storefront. The Registered Customers group represents customers that are registered, but not necessarily logged in.
Customer groups are assigned to a campaign. Promotions inherit customer group assignments from their parent campaign. When defining a promotion, you can remove all customer group assignments (not individual customer group assignment) from the Promotion configuration page; but you can't add unassigned customer groups.
If an active promotion inherits a customer group qualifier of registered customers from its active parent campaign, the promotion is automatically added to the shopping cart of registered customers.
See Creating a Campaign and Creating a Promotion to learn how to assign a customer group to a campaign or promotion.