You can add your Client's Site Name onto the invoices that you send to your clients by updating the address format in your Account settings on the Web Portal.

Web Portal > Settings cog > Accounts > Invoices > Address Format.

This will also flow through to your Quotes. 

Please ensure that it is entered as sitename for it to work correctly, invoices that are already created will need to be regenerated for the sitename to appear.  View PDF > Regenerate.