While the app's a work in progress (and if you forgive the shameless shill), Splitsies (http://splitsies.net) does more or less the same thing and is freely available in the App Store.
Doesn't do groups yet (because I haven't worked out how to make them awesome yet) but its coming...
Doesn't do groups yet (because I haven't worked out how to make them awesome yet) but its coming...