Collaborators - Enterprise Pinnacle Subscription

Our design department receives 'Brief Forms' that we have created from all over the world. Most of these briefs are raised by collaborator license holders, but the very restrictive access is causing us real problems. Ideally we could do with them being able to Guest Review, Share Briefs (that they themselves have created) and add subtasks. Am i missing something? I have looked into access roles in the hopes I could add something there but it seems all they can do is raise a brief and that's it.