No ... I don't think you would need roles and permissions for the task project but since you were enthusiastic about the user and groups I thought I'd pass along a solution for a comprehensive (but external) user management system.
EDIT: I also should have mentioned that we're updating our original system to be compliant with the specification and open sourcing [1] the code that's not custom to our application.
As far as I think, it is straight forward. But I also agree when you work in a complicated hierarchy, you probably need these systems.