- Have a separate partition with "Ignore permissions" checked in the Get Info box.
- Make sure you have synced all Address Books via import-export.
- Copy the Address Book folder you want to use as the master from /Users -> user_name -> Library -> Application Support to the other partition.
- Delete the original folder for each user (after verifying the copy), and make a symbolic link from the new location of the master Address Book folder. Symbolic links are made in the Terminal: ln -s path_origin path_alias.
- For each additional user: Delete the original folder and make a symbolic link.
[robg adds: I haven't tested this one.]

