We have a couple of options in defining Customer Groups.

STATIC CUSTOMER GROUP

You can create a Static Customer Group, which defines the name of the group and what members of this group get (discounts as set in a Price Book or VIP status). 

FREQUENCY BASED CUSTOMER GROUP

You can create a Frequency Bases Customer Group, where customers are added to the Customer Group based on qualifying for the group. Multiple levels can be defined, as well as the date range for qualifying.

Navigating to Customer Groups - Configuration: