1. Goto your Tandem Console and click on 'Manage Domain'
2. Click on 'Add Card' button, a new window will launch to enter your card details.
3. Enter your card details and Click 'Submit'
4. In order to pay for external users from outside your domain, enter the emails of the external users and add them to the billing account.
5. The External users are added and can be removed by clicking on 'X'