I have an event that is normally $25. Groups of 10 or more will be discounted to $20 per person. I don't seem to see a way to do this with the built in group functionality.
The suggested post is not what you need. This is easily handled without the need of a custom field. When you create a new event (or copy an event), you'll see 10 rows of pricing options. You would need two.
1 person $25.00
10 people $20.00
The price is always calculated per person, so when you set it this way, 1-9 people will be charged $25/each. 10 or more people will be $20/each. Our video tutorial for event setup explains this in more detail if needed.
Note, you will NOT see the various group pricing options after the event is already created. You'll need to re-create the event in DT Register.