Haiku already has support for multiple users at the kernel and filesystem level. You can `useradd`, `passwd`, and then `su` (or even SSH into your newly-created user on your Haiku install from another machine), and also `chown`, `chmod`, etc.

What isn't properly supported yet is running GUI applications as anything other than UID 0. Adding support for that will require some careful refactoring across a few different components. Actually, if you added a handful of hacks in a few places it might work already...