This can already be done by default with Plex. It does require a Premium Plex account however. I don't think you can setup multiple users on a Plex Free account.
Your idea is good, it would simply require the host of the system to manage setting up the accounts to said users directories with their given email address, then having them assign pin/passwords to their "guest" accounts to avoid anyone waltzing into their library. Additional to this, there would need to be multiple Libraries setup. So, user1TV, user1Movies ... etc etc. and them only having access to those libraries for reading and writing.
It's not a bad plan, it just requires manual intervention. Of course, if you only have 3 friends on a server... this is a simple task, then the 3 of you simply split the cost of the Premium Pass. They are only $4.99/monthly and $39.99/yearly.