Hi, I'm using Mepis with KDE and I was wondering is there a way to force the log-on screen to come up when someone comes to use the computer after it's been in inactive/screensaver mode?

Sure, when the screen saver is activated and password is enabled to come out of it, the only user password that will bring the screen saver down is the one who was logged in when the screen saver was activated. To enable screen saver password, open KDE control center and in Appearance & Themes, click on screen saver and check the box next to "Require password to stop", and set a time limit, the default may be 60 seconds which means that if the screen saver has only been activated for 45 seconds, no password is required, it it has been activated for over 60 seconds, a password is required to bring up the desktop and disable the screen saver.

EDIT: Apparently I was slightly wrong , you can also switch user from that screen saver login.

Thank you specially for your EDIT at the end. I want the ability to switch user once you get out of screen saver mode.