In order to add or remove users, you must be an Organizational Admininstrator ("Org Admin") for your account. Follow the instructions below to add or remove users from your account:
Visit your organization settings page and select the users tab. Select the Invite User button and fill in the required information to send a new user an invite.
You can select which of your active account subscriptions the new user has access to by toggling the sliders at the bottom of the invite user page.
When done, hit invite to send the invitation.
If you'd like to edit their permissions: locate the user, and click the edit icon on the right side of the page, and it will allow you to edit which products they have access too.
Removing a user:
Log into your Organization Settings page, and select the users tab. Locate the user, and you can choose to either edit their information, or remove them by selecting the trashcan icon.