Hi Fotis,
I am really surpirised with your comment as 'why to switch back, when you have k2'.
Have you any time figured out how could it get difficult when, there are multiple user groups & there is no tut on this, not even any hack. This is like getting a FULL STOP in my current project, only because of not being able to create multiple user groups. The joomla core provides this. and anybody would have to fully stop k2 and get back to joomla core, no one would only use the joomla core user & user groups, because problems would rise.
Please reply to me & correct me on the above & suugest me what shouold i do, go back to joomla core with multiple usergroups or stay on k2 without the same feature.