In the 'Users' section you will find an overview of all users in your account (1). In the 'ACTIONS' column you will find a button to remove users from the account (2). If you want to invite new users to your account, type in their e-mail (3), and press the 'Send invitation' button (4). After sending the invitation you'll be able to see the pending invitation until it's accepted by the user (3).
To find out more about how to customize your account see the article about General account info settings.