We've been contemplating multiple people responsible, but haven't built that yet. For now you can assign one responsible to a task. However, that doesn't mean you have to be the only one working on it. Through the task-chat you can collaborate with others, and nothing stops multiple people from working on it. We may add options to assign multiple people technically speaking, but I'd say for now simply jump on the task but leave a comment that you'd like help etc. Does that work?
The URL is to point people to a single primary resource --- sometimes it makes sense (e.g. if the task is REVIEW THIS PARTICULAR PAPER), and sometimes it doesn't (and in that case it can be left empty).