To set up a billing group where you have multiple accounts all billed to a master account follow the steps below.
Setup the Master account
Open the customer/account that should be billed for all of the group's sales. Click on the name and select the Admin tab. Change the Account Type to Group Master.
Select Master Billing from the group types list and click Save.
Now go to all accounts that should have their sales billed to the master, and add them to the group. Click their name, select the Admin tab, click the group dropdown and select the master account and then Save.
It's very important to add the sub-accounts to the master before creating any orders. Any orders created before the sub-account was added to the master will need to have their payment account changed. So it's best to set up your billing group before you begin processing their work.