Mailtrack is a Chrome browser-based service and like other browser-based services (e.g. bookmarks, stores passwords, etc.) Mailtrack is available for everybody that uses the same Chrome browser in the same computer. Unless you create a separate individual profile for each user, the Chrome browser can't know who is using this computer, so all Mailtrack notifications will be displayed independently from the user.
In order to avoid trouble with privacy issues Chrome browser allows you to create separate individual Chrome profiles for each person that uses Chrome in the same computer. Learn how to do this here