We've added an enhancement to the Customer Groups tab that allows you to easily split a Customer Group into multiple groups. You can find this setting on the Customer Groups page:
When splitting a Customer Group, you can search by name or filter by email domain to find customers you want to select for the new group.
Both resulting Customer Groups will retain all settings of the original. If your Customer Group has a Pricing Plan for price overrides or automatic coupon application, this Pricing Plan will be applied to both resulting Customer Groups as well.