I actually had the exact same issue when trying to set up users on a no brand tablet. My tablet was rooted out of the box, so I installed Xposed and then the module Multiple Users for Phone. Even though the title says it's for phones, it works flawlessly on my tablet. It adds the option
Users to your settings page.
Info about Xposed from the installer page:
Xposed is a framework for modules that can change the behavior of the
system and apps without touching any APKs. That's great because it
means that modules can work for different versions and even ROMs
without any changes (as long as the original code was not changed too
much). It's also easy to undo. As all changes are done in the memory,
you just need to deactivate the module and reboot to get your original
system back. There are many other advantages, but here is just one
more: Multiple modules can do changes to the same part of the system
or app. With modified APKs, you to decide for one. No way to combine
them, unless the author builds multiple APKs with different
If you want clearer instructions on how to install, comment and I'll edit the question. I can't link to all the links because I don't have 10 reputation (yet).