Can You Create Sub Groups in the Contacts App on an iMac?

Actually, it is possible to do:

1. In the Contacts app, create the groups for your purpose, e.g. "Golf" and then "Golf Buddies", "Golf League", etc. You can do that by clicking the little Plus (+) button
2. Once you have added these groups, they appear in the side bar of the contacts app. The side bar does not allow you to display them hierarchically, they appear to be all on the same level (top level).
3. Drag the Sub Group on top of the Parent Group. In this example, drag "Golf Buddies" on top of "Golf". The dragging will be indicated by a little file card icon. Please note you will not see the Golf Buddies list in the sidebar move indented. It will stay as is, however, it will be part of the Golf group.
4. To verify, click on the Golf Group in the sidebar, and you will see that the sub group "Golf Buddies" is actually now part of the Golf Group.
5. Add any additional groups as you see fit. You can nest them even multiple levels.

No.

You're welcome.

I stand corrected.