We are looking at using Plex but from the documentation so far we see mutiuser/home and watched show support. However the one thing that is not obvious is tracking who has watched which show… say each of two people in the home have an account but they sometimes watch shows at the same time but not always and want to both login so it correctly tracks this. Having some way in the preplay show window to select multiple “in thr room” accounts would save lots of manual futzin around with symlinks, kibraries etc… right now we just use the file manager and directories but if plex can automate this thst would make it much easier.
This is not possible.
There can be only one plex user logged in at once. The ‘watched’ status is registered for this user.
To enable the functionality you desire, you’d have to select before/after each watch, which user(s) was present in the room. I don’t think you want to do that every time.
Moreover: If one user decides to leave in the middle of playback, things get complicated very fast.
For such a system to work satisfactorily, you’d have to have a camera with face recognition above your tv screen which registers every person present in the room separately.
I am unsure whether I’d want that…
I have one of those… It’s called a Kinect and works great! 
Everyone at Redmond knows the color of my briefs for sure.
Perhaps a feature you can turn on so that when a show finishes up; it pops up a dialog asking if you want to mark watched for ‘this user’ or ‘all users’. That way people who don’t care leave it turned off, and those with wives, turn it on. 
