Hi Adriaan,
You can view a list of your employees from within the Management Interface by logging in as the System Administrator and going to Settings | System Settings | Organization | Employees. Here you can see all of the employees listed alphabetically, along with their role and last login date and time. These two can help you determine if an employee is currently utilizing the system.
You can also use the Web service call, GetAllAgents, to retrieve a list of all users that are marked as agents in SmarterTrack. Here is the link for the SmarterTrack Web services where you can find more information on this:
http://portal.smartertools.com/services2/
I hope this helps!