Application Wizard comes with a set of seven predefined group icons, but you can also assign custom icons to groups if you like.
Open the Application Wizard pane in System Preferences and click Groups.
Select the group in the list and choose Other from the Choose drop-down menu.
Select an icon from a custom set or an ICNS icon file or select any of the items (disks, folders, applications, documents, ... ) on your computer to use its icon. Click Choose.
Great icon collections can be freely downloaded from Iconfactory.