Creating New Customer Groups with Excel Import
You can simultaneously create new customer groups and add customers to them by using Excel import:
Add a new group:{group_external_id} column to the Person Customers.xlsx Excel file, with {group_external_id} as the name of the new group.
Note: No group with the same name should exist in Back Office beforehand.
Enter TRUE as the value in the column for all the customers you want to add to the new group.

Creating a new group with the name NEWGROUP & adding members to the group
When you save the file and import to the Back Office, this will create a new group whose Name and ID are the same as group_external_id; the group will contain all the customers who were included in the group in the Excel file.
After the new group has been imported to the Back Office, you can modify the customer group settings if you want to change the name and/or ID.