Contacts sidebar
The Contacts sidebar lists available lists, folders (if you use an Exchange account in Contacts), and network directory services.
Show or hide the sidebar: Choose View > Show lists or View > Hide lists.
Resize the sidebar: Drag the sidebar’s separator left or right. If you drag it all the way to the left, the sidebar is hidden.
If you don’t see expected lists, folders, or directories in the sidebar, choose Contacts > Settings, click Accounts, then make sure your accounts are enabled.