You must be an admin in order to invite or remove people as users to your portal.
As an admin, navigate to the "My Account" button in the top right hand corner of your portal. Scroll down and click on the“Manage Users" button.
From here you are able to to add new users and specify their permission levels. Users do not need to have a domain specific email address to be added to your account.
To remove a User, press on the User's name in the list of users. Press Delete on the bottom of the Edit User pop-up.