First, click your name in the top-right corner of the application and select "Account":
Then click the "Subscriptions" tab to manage your users. To add a new teammate to an existing seat, simply type in their email to the open email box and click “Send Invite.”
To add a new teammate with a new seat, select either “Add Full User Seat” or “Add Limited User Seat,” choose between monthly or annual billing (which saves 10%!), and enter their email.
Once your teammate has confirmed their email and entered their info, you’ll see their name on your Subscription page.
To remove a teammate, select the “Remove Teammate” button. Note: This removes the person’s access, but does not remove the seat. If you paid for a yearly subscription, you will have this seat until the end of the billing cycle and ought to consider putting another person in this seat. If you paid monthly, and you don’t have an employee soon replacing the one leaving, you may choose to cancel this seat to cease billing.
Need more info on billing? Read this article.