I have been forced to use two separate tools for managing my work: Omnifocus because I want its GTD structure and Process because my work often involves projects where there are multiple people assigned to particular tasks. I wouldn't have to do this if Omnifocus would let me create additional columns, among which I would include a "persons responsible" column. I don't find it helpful to manage people as contexts so I'd prefer to be able to filter my project list by particular people as needed. I once used iGTD which had a really slick integration with the Apple Address book that may this a dream. I gave up on it because it hasn't been supported by its developer for quite a while.