How to control the list of people shown in the "Search contacts ..." box


The contacts app is disabled for most of my users but when I log in as one of them and click on the "Contacts Button (top right next to the user menu a list of the other NC users is shown. For some of them only users out of the same group are shown others can see everyone even though they are in a dedicated group as well.

How can I control what is shown there or is there a way to completely disable that feature?