First, you must have access to a FireCloud Billing Project. All compute and storage costs in FireCloud are tracked through FireCloud Billing Projects, which transmit charges to Google Billing Accounts. When you create or clone a workspace in FireCloud, it is linked to a single FireCloud Billing Project that tracks all storage and compute costs for that workspace.
To set up a new billing account, check out this article for step-by-step instructions.
To add a user to an existing billing project, check out this article.