A client is a person or organization for whom you do work or otherwise associate with your projects, invoices, payments, and retainers.
You can add contact records existing in your Contacts app as clients in Billings Pro.
To add contacts:
Choose File > Add Client from > Contacts... In the sheet that opens, select a client.
Alternately, you may click the "+" button at the bottom-left corner of the Billings Pro window and choose "Add Client from Contacts..."