The "Show Users By" is a portlet that shows users based in some criteria such as the user's Role, Job Title, Group, Category or a Tag.
This filter works in an static and a dynamic way, the information shown by this portlet is the Name, Job Title, an image thumbnail, the email (optional) and the main phone (optional).
This portlet is a good example of "Inter-Portlet Communication" as it allows communication from other portlets to filter users in a dynamic way and to other portlets sending information of the selected user.