In order to apply a charge to a group you will need to create a partner category.
Login as Admin and go to the Configuration menu located on the top of the screen.
Configuration > Partner Category Setup
Once inside, double click to create a new category. Name it whatever is relevant.
Save and close.
Then go to the trading partners who are included in this group and add it to their record. Select it under CATEGORY. Save and Close
In Charges .. select the category under Supplier Category.