You may be interested in a project I've put together which syncs OmniFocus and Toodledo (a web based Todo service). Since Toodledo allows shared projects between users, and these shared projects sync back to OmniFocus, I think it could work for you. There is also a pretty good iPhone interface to Toodledo. Its in late beta.
http://groups.google.com/group/oftd
-P