When you create your administrator account, you choose to be billed either monthly or annually. At the time of sign-up you can choose to either add team members on to your account at that time, or add them later. Two charges will appear on your credit card: one charge for your admin account and one charge for all of your invited users that you add at the time of sign up. So, if you bought one admin account and two team member accounts at the time of sign up and are being billed monthly, you will see two charges: one charge for $10 (admin account) and one charge for $20 (two invited user accounts).
You can add additional users at any time, and each time you add additional users, they will appear as separate charges on your card. So in the example above, if you then added three more users one week later, you will see a separate charge for $30 on your card repeating monthly going forward. If you bought the original account in this example on the first of the month, going forward you would see one charge for $10 on the first of the month, one charge for $20 on the first of the month and one charge for $30 on the 7th of the month, repeating every month. We bill this way so that you have more control over when you add users to your account.