you can create users through the Karaf Console. There is currently no way to do so through the UI yet. This is briefly touched upon at Getting Started with OH3: rewriting the tutorial - 2. First steps. I don’t know the specific commands but this link and what it links to should get you started.
BasicUI should be largely unchanged. I don’t think access to items its locked down based on user but I’ve not played with this aspect of OH 3 yet.
But the way this works has nothing to do with how the items are defined. It doesn’t matter if they are defined with .items files or though the ui.
Look into the model options that key you add equipment from thing and add points from thing. I find that a very rapid way to add lots of items.