You can add new users in Kubuntu via graphical user interface from within the System Settings. This is basically the same for other KDE-based Linux distributions such as openSUSE as Kubuntu itself is based on KDE.

Follow these steps to add new users in Kubuntu and KDE in general;

1. Open System Settings by going to Computer→Applications in the start menu.
   
2. Click on Account Details in the Personalization section.
   
3. Click on User Manager on the left navigation tab.
   
4. Click on New User.
   
5. Fill in details such as the username and Real Name.
   
6. A window will popup upon clicking the Password field.
   
7. Enter a complex-enough password till This password is good text appears and click Ok.
   
8. Tick on the Administrator checkbox if you want to allow sudo or administrator access for the user and on the Automatic Login checkbox if you want the system to automatically login to this user upon boot-up and click Apply.
   
9. Enter your password to authenticate to the system.
   

   This require your user to have administrator privilege in the system.

   
   

10. Your new user creation is now complete.