Two identity folders automatically exist in the Address Book: the Main Identity's Contacts folder and a folder that allows you to share contacts with other identities—the Shared Contacts folder. Neither of these folders can be deleted (although folders for additional identities will appear as new identities are created).
You can make a contact available to the other identities by putting it in the Shared Contacts folder. You can create a new contact in the shared folder, or you can share an existing contact by moving one of your contacts into the Shared Contacts folder.
If you cannot see the Shared Contacts folder on the left, point to the View menu and select Folders and Groups.
This will allow other identities to see the contact.