As super admin, you can show the list of tenants using the tenant admin console (refer to TenantInterpreter.java). To get a list of all people/usernames you can use the REST interface "/api/people" (refer to People.java and PersonService.java [getAllPeople]).
In a similar way, you could write a Java-backed web script to access the TenantAdminService and PersonService. To get all the users under each tenant would require looping through each tenant and switching context when getting the people (refer to AuthenticationUtil.java [runAs]).
Regards,
Jan