For basic information on creating an invoice see: How to prepare an invoice
In this article you will learn how to add an additional person or business (internal/external) to a Client in order for you to be able to send an invoice to them.
Creating additional contacts for your clients
In the left-hand menu click on Clients and choose List
Once the list of Clients is displayed choose the Client you wish to invoice
Now you can choose additional Contact while creating an invoice
You have now basically created an invoice for someone other than primary contact of the client.