If you wish to add a contact to a specific company, simply open the company home page. In the 'Actions' pane you will find a link to 'Contacts'. Click here to open the contacts section for this company.
The contacts section is a simple address book for your companies. Click 'New Contact' to create a contact. This will open the contact dialogue box as shown below.
The page allows you to add all the contact details you would need, such as name, phone numbers, address and so on.