The Open End
At one end of the spectrum, you can ask your students a question or give them a problem or an ultimate goal and then leave them to do it their way. Your students would then be free to decide who's part of their group, who gets what duties within the group and how often and when the group will meet. They will decide what material they would need to access and how to go about getting the information they require. They will tell you when they've fulfilled their task and what form the final product will take.
You really can't be too specific when you set the objective for this kind of exercise. You would, of course, give them the overall goal and the deadline and tell them in broad terms what you expect them to deliver at the end of the process. Your focus would be more on the process of the collaboration than the final outcome in this totally student-centred learning process. You would observe and facilitate the students' interaction with their peers and the learning environment and you would encourage them towards self-actualization through the personal and social change that they will undergo during the collaborative process. However, you will not exert any authoritarian control over your students.
In this kind of collaborative activity, your role is limited to that of consultant and advisor when your students need your input. You will help them when they ask and you may offer suggestions which may or may not be incorporated by the group. At the end, it will be your job to assess the final product, again encouraging the group to consider and process your viewpoint. Your assessment, in this case, is likely to be more qualitative than quantitative.
If the activity does what it is supposed to, you will have a group of extremely motivated students who would have appreciated the trust you've invested in them. In the best case scenario, your students will be completely involved in the learning process and will own their task with total responsibility. The outcome of the collaborative learning process will exceed your expectations.