I can imagine a couple of different ways to modify the flow for that purpose as long as those users are not using a single instance of Node-RED at the same time, in which case they will visualize the same show.
Could they self-identify via a text input and if the username is validated, then it launches the proper flow sequence? In any case, this would work only if clients are not accessing at the same time. You can use the ui-control node to monitor when a client "disconnects" and stop the auto tab shifting and return to the login page.
The fact this is possible is exactly why I love Node-RED. Right now I work with a school that has a display screen in their entrance hall, and it uses some arcane server software to pull from different sources and display various data (school blog, twitter, bbc news, etc.)
Unfortunately it's riddled with bugs and barely manageable, so I'm planning on implementing this in Node-RED
This modified flow is a step towards a better solution. It allows to configure several "show" sequences as well as to trigger them from the editor. Needs to be improved if the idea is to control the tirggering remotely.
the ui_control will report a socketid of a connecting client - commands sent back with the socketid intact will only go to that client - so you can direct things to a certain client... the trick of course is to associate the random socketid with the required screen.
(There is also socketip which shouldn't change so randomly - but it's the socketid that directs the messages.)