How do I determine active users?

If you have access to the server side and the environment is configured with floating licenses, one might leverage the repotools-jts.sh -listIssuedFloatingLicenses.   I do this to trend license usage patterns over time as well as collect information regarding the collection of users based on "business unit"

From the client side, one can look at "Recent Events" for a user ( right click user in the Team Hiearchy / Show Recent Events .. ).  That only populates something if the user has made some sort of change, not if they login and take a look around.

Other means depend on what information the J2EE application server can emit.   Tomcat can be configured to have an access Valve which records HTML activities with the user ID.   WebSphere must be configured to do Auditing.  Not sure what WebSphere Liberty can provide.    This data is available on the server side and one must use some sort of analysis to ferret out the desired information.