Each Customer may have multiple contacts assigned.
To add, edit or remove a contact, go to the Customers menu, and click the [Manage Customer] button on the customer you would like to make the change to.
This will open the [Customer Profile] screen. Switch to the [Contacts] tab to manage the customer's contacts:
Each contact can also be assigned a [Contact Type], which will be used to control what information/functions will be available to that customer: