quantity discount for unique items

I am developing an ecommerce site for event registrations. You have already helped me attach an attendee to each registration but now I have another issue.

The company gives a $5 discount off of each registration when you register 3-5 people at the same time, $8 off each registration when you register 6-9 at the same time, and $10 off for 10-20 registrants.

I tried using this calculation/discount as you explained in another thread:

TotalItemDiscount:
((abs([Quantity] >= 3 && [Quantity] < 6)?5:0) + (abs([Quantity] >= 6 && [Quantity] < 10)?8:0) + (abs([Quantity] >= 10 && [Quantity] < 20)?10:0)) * [Quantity]

then define the discount on the discount tab as:
Trigger: Number of unique items in the cart

Calculation: Based on multiple of column subtotal

Subtotal of column "TotalItemDiscount" "times" "1" "plus" "0"

However, it doesn't work in my situation because the registrations are no longer grouped together because the different attendees make them a unique and separate item.

I hope you can help.