Here's an AppleScript that will do show all groups belonged to by the selected person
tell application "Address Book"
set nameList to {}
set thisPerson to selection
set groupList to properties of groups of (item 1 of thisPerson)
set numberOfGroups to count of groupList
repeat with i from 1 to numberOfGroups
set groupName to (get name of item i of groupList) as string
set nameList to nameList & groupName
end repeat
return nameList
end tell
If you're not familiar with AppleScripts, launch Applications -> Utilities -> AppleScript
copy and paste the above script into the AppleScript editor
click the <Compile> button to verify syntax
Now, to run the script, Open the Address Book and select a contact from "All Contacts"
now run the script.