Collaboration To Do's / GTD / Project Management Software for Mac

I have been looking for a few hours for some software to handle project management and to do's for a small number of staff (about 5) all on macs.

I have personally been using Things for Mac (and syncing it to the iphone app) and it works really well. However, it does not (yet) have the facility to delegate tasks to staff members where all the people involved can see, update and log the tasks entered.

Ideally, what I am looking for is a app like things for mac which will let me add personal to do's and then assign collaborative to do's to staff members. Then, when they have completed those tasks, they log them as complete which will update my list over a network / cloud / internet type system.

I have looked at the syncing facility for thing for mac which would enable me to sync job lists with the other macs on the network. However this would mean that they would have access to all my to do's as well rather than each of us having personal lists and collaborative lists.

I have looked at omnifocus as well and I don't think they have this functionality either?

Is there anything that can do this or are we going to have to go for a web based offering?

Yeah, I have looked at web based systems such as basecamp but would rather a desktop app just out of ease (no logging on etc).