There are Three types of accounts: Starter Account (free), Business Account (paid), and Enterprise (paid). If you upgrade to a Business Account account, you will unlock additional features, including the ability to have multiple users. Billing is performed monthly or annually via credit/debit card charge. The billing information is set via the Administration Tab, Billing section. Upon successful payment, you will receive an invoice. You an also view the historical invoicing via the Billing section.
Please contact firstname.lastname@example.org if you would like to upgrade to an Enterprise account.