To link a billing item to multiple groups, you first have to enable the setting “Billing item links to”:
Go to: Reg. Site -> Billing module -> Payment settings -> General settings
Set billing item link to “Groups” and save the changes.
Go to: Reg. Site -> Billing module -> Billing items
Select or create a billing item. Select link to “Attendee groups” and then you can assign multiple groups to the billing item as displayed below.