Billing accounts are nothing but user profiles that is created in the tenant portal by the customer. The tenant subscriptions are mapped under each billing account. The service administrator can assign user roles to the billing accounts based on which, when a user logs into the tenant portal, all the subscriptions that are assigned to the billing accounts to which the user has access can be seen under the Usage and Billing tenant extension.
Click 'Billing Accounts' menu under 'Manage Subscriptions' section.
Click on the desired billing account.
Click on 'Add' button to add new user role.
Click on 'Save' button.