To access this feature you must have administrator rights.
Connect to the administration, then click on Tools > Monitoring.

You will access a screen which summarizes all the activity of your company in real-time with:
Session start time
Session owner
Project name
Ram use (in Mb or Gb)
CPU use in %
You also have the possibility to select a priority to each session (High, Normal, Low) and to kill a session if you want to free some resources for example.