You can manage you users by the “Users → Users” link in the top menu. Please note that the are 2 types of view:
If you are default respondent you can manage the assigned to user managers and see the whole list of the users. Also all of the created by mail threads are assigned to you. To set the default respondent you should go to the profile page (“Users → Administrators” in the top menu) and set the appropriate checkbox in the “Integrations” tab. Also on the “Managers” tab you are able to set the list of the managers who will be able to communicate with this user.
If you are not default respondent you are able to see only users assigned to you.