Interesting. When I search iCal Help using "groups" the following is shown.
"Organize your calendars in groups
You can group calendars so that they appear together in the Calendar list.
NOTE: You can only group calendars under the heading On My Mac in the Calendars list.
1. In Calendar, choose File - New Calendar Group.
2. Name the group.
3. Drag calendars to the new group in the Calendar list."
However, my File menu only has New Calendar listed, followed by iCloud. I have tried turning off enabling of the iCloud account, at which point all of my calendars disappear.
If Calendar Groups are not available in this new version, then I suppose the only thing to do would be to export everything, and re-import into one calendar.