How Do I Manage Projects That Are Shared?

How do i manage projects that are shared with other people? This is a common one, because we are getting a lot of people these days trying to manage things in the clouds. Who owns the project? There needs to be an ownership of the project. What ofe the protocols about people in putting to that project? How are action items delegated on that project and how does all that come together? And at what point is a project so big that it would be useful to create a separate sub project?