Trying to add new billing manager but getting message saying not in contact list

Hi I'm hoping someone can help,

I am trying to add a new user to our business account.

I go into Add a new user and enter their name, email address, confirm it and set their permission level.

It waits for a moment then returns back saying the email address could not be found in the billing manager contact list. Please try again

This happens regardless of which permission set I try to use.

Where is the Billing Manager contact list held so I can add our accounting deparment.

Thank you

Gordon