I sure hope the multi-user aspect of Plex is redesigned.
Here's my take on it.
Plex/XB1 should be able to use the "Users" capability on the Plex Media Server, in much the same way as the Netflix app does. Even better, it should be "saved" for each user. and if multiple people and multiple PMS/Users are detected by XB1 as present (through the Kinect camera) it offers multiple choices of User login.
For a family of four:
Dad - Has XB Live, Xbox GamerTag, PlexPass
Mum - Xbox GamerTag
Kid1 - Xbox GamerTag
Kid2 - Xbox GamerTag
One XB1
One Plex Media Server configured with PlexPass
Plex Media Server is configured with two users:
Parents
Kids
Initial setup on XB1:
Dad signs in to XB1, Starts Plex/XB1, connects to PMS, logs into "Parents" user. Dad signs out..
Mum signs in to XB1, Starts Plex/XB1, connects to PMS, logs into "Parents" user. Mum signs out.
Kid1 signs into XB1. Starts Plex/XB1, connects to PMS, logs into "Kids" user. Kid1 signs out.
Kid2 signs into XB1. Starts Plex/XB1, connects to PMS, logs into "Kids" user. Kid2 signs out.
Scenario 1:
Mum starts Plex. She is the only one signed into XB1. It automatically shows the Parents shows.
Scenario 2:
Kid1 starts plex. He is the only one signed into XB1. It auotmatically shows the Kids shows.
Scenario 3:
Mum and Kid1 are signed into the console and Plex is started. It gives an option to select either Parent or Kid Shows. Any TV shows/folders below a given rating are hidden but can be unhidden through the in-built XB1 unlock system.