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I might use some variant of

parent task is root task of containing project

Code:
tell application id "com.omnigroup.omnifocus"
	tell content of front document window of front document
		set lstTasks to value of selected trees
		set oTask to first item of lstTasks
		tell oTask
			set blnIsInGroup to (parent task is not root task of containing project)
		end tell
	end tell
end tell