private notes is very usefull , but I miss the functionality to restrict visibility to a group of users. Is there a way to realise this in civicrm or is it necessary to make a new extension?
You might want to try this extension: https://github.com/AlainBenbassat/eu.businessandcode.notepermissions
It creates a permission in your CMS for each note type you create in CiviCRM. You can then selectively assign these permissions to users who have to see the corresponding notes.
The extension has not been extensively tested, so I'm looking forward to user feedback.