It's been confirmed a few times on this forum already that the current method of adding/managing users is overly complex, but here I am going to propose a KCM UI that would be primarily for Unix variant users, since Windows and Mac OS (yes, I know it's a Unix variant as well) both have quite affective GUIs for user management. This UI is to make the power of Unix's group system easily accessible when creating a new user.
I made this in QtCreator, so the .ui file is available if you want to make changes.
Madman, proud to be a member of KDE forums since 2008-Oct.