You can get a list of active users seen by the Diagnostics server in the last 60 seconds. And you can see the Queries/sec indicating how much load the user generates with summary or trend queries.
From the main Diagnostics UI select Configure Diagnostics and the Components page is displayed. (You can also access this Components page by selecting the Maintenance link in any Diagnostics view). Select the query link and then select the Active Users link at the bottom of that page to display a list of active users. Also this data is under Mercury System groupby.