This is offered via Liferay component itself. It's disabled OOTB because of performance reasons.
live.users.enabled=true
Set the above in portal-ext.properties and recycle USS
then: http://host:port/group/control_panel --> Portal (on the left pane) -> Monitoring
note: this is a liferay component itself and is disabled OOTB as they noted performance issues around it
_R