Billing subusers can modify anything billing related. This can be anything from paying invoices, to cancellation requests.

Go here, you will have to login if you're not already.
Find the blue button button titled 'Invite New User'

Enter the email of the user you wish to add, and select the permissions you wish to grant them.

We are not responsible for any damage caused by sub-users. Make sure you only give permissions to users you trust.
The subuser will receive an email with information about creating an account if they don't already have one.