An ambitious idea. Some questions immediately spring to mind.

> Task is owned by a Project. Project belongs to a User. User is a member of Domain.

Can Tasks have subtasks? Recursively? To an arbitrary depth?

Can Projects have subprojects? (see above)

I assume since a "User" is just an email address, a group email address would implement a "Group" at the "User" level?

In a company setting, Users (Contractors, Employees) come and go during the life of a Project. What happens to Tasks assigned to a User who leaves the (company|Domain|Project)?