What if the primary representative of a party is not the person who is responsible for paying the bills. How do I change that?

Often, the mediation invoices should be directed to someone other than the primary representative of a party.  In your Case Setup, you add the person responsible for billing by selecting the Add participant affiliated with this party button...

...enter that person's information (including the billing address, phone number, and email address, which are required to generate an invoice) and Save it. 

Then in the main Case Setup screen, select that person's name from the Responsible for Billing dropdown list.  

Read more in Getting Started with Billing in ADR Notable.