ZenGengo offers the ability to any Plus level user to monitor the number and names of connected users for any of your assignments in real time. This functionality is called "Live Monitoring".
To use the Live Monitoring tool, first click "View" on the "Submissions" dropdown menu of the assignment you would like to monitor:
Next, click on the "Live Monitor" button:
You will now be able to see a list of students who are currently engaged with the selected assignment: