Hard to guess what users install....
But I presume you did not use the stable ISO, but one of the test ISOs to get ready for the plasma 5 move.
Note exact name and date of ISO used.
The application you use on KDE 4 (and available in KaOS stable ISO), is kuser. It is not ported yet to kf5, so for now you will have to use the "useradd" command to add another user.