You can add individual contacts manually, from .CSV or .XLS file and from Apple Address Book.

An individual contact can be added via the ‘+’ button above the left contact list. A popup appears where all relevant information can be placed. In the field ‘Contact type’ you are able to indicate which type of contact you would like to add. For instance a company, supplier, personal friend, prospect etc. When finished, click on the save button and your new contact will be added and you will be lead to the contact’s card.

How to import a bulk of contact from file or Apple Address Book can be found here.