Not sure if you found a solution to this, but here is a link to a free update to upload to your website so you can display all the authors in a K2 User Group.
www.jiliko.net/extensions/downloads/free/10-users-view
DON'T use the Joomla Installation to install it. All you need to do is use FTP to go to your root directory, unzip the package and copy the files into the root directory.
It will update your K2 so that when you go to make a Menu Item, you have the option to display a User Group or multiple groups.
Hope this helps.