For reasons unknown, everything just completely vanished from my Contacts (including contacts and all of my groups). Note, these are all Contacts that were stored on my Mac -- I don't use iCloud and they are not synced anywhere else.
No problem -- I have a Time Machine backup, so I enter TM, scroll back in time to yesterday, and there they are. I select all and restore all of my contacts. Back in the present, all of my individual contacts are there. But none of the groups are.
I've just spent several hours fruitlessly trying to figure out how to restore my groups. One thing that has not worked is copying the entire Application Support > AddressBook folder from my Time Machine to my user library: when I try this, and then restart Contacts, the entire list is empty again. (I don't understand this behavior at all, by the way.)
Where does Contacts store my group metadata? How can I restore it from a Time Machine backup?