This capability is already built in to maya (I am currently using maya 2012. Not sure how far back it goes). To access the set manager gui select...
windows > relationship editors > sets
This brings up a gui that will show you all your sets, what is in them, and has a + / - button to add and remove elements to and from selected sets.
"If I have seen further it is by standing on the shoulders of giants." Sir Isaac Newton, 1675