How to delete a Calendar Group?

Using Mac OS X 10.11.6, "El Capitan", with "Calendar app" Version 8.0,

I found that selecting the menu item...

File -> New Calendar Group ⇧⌘N

...will create an empty new entity in the "Calendar app" window's left sidebar, initially named "Group", but renamable. Selecting that menu item three times will create three similar entities.

Well, fine.

Now, how can I delete them?

--Gil