Unless I’m mistaking, there is currently no way to track a User. You have to track players (and thus the User) and that is a constantly moving target.
I notice the Plex server sensor actually contains the data, but is inaccessible. I explain it here.
I propose an additional attributes to the Plex Server Sensor for Active Users (or a completely new one for each user)… something like…
[User] - [Player Client]: [Show/ Artists Name] - [Season / Episode / Track Information]