In the Navigation menu click on "My Account" > "Users".
This page displays your current users and how many still available to set up.
Click the green + button to add new users (as shown below).
If you click the "add new user" button and you used all your available user logins, you'll be prompted to pay.
You will then be prompted for the new users details. They then get an email with their own username and password to access the account you are currently logged in to.