I don't think we have an option to work on one story by group of people at the same time. Usually when user creates a story that tags that user and it will be only seen/can update by the user who created it. Other users wont see that story unless owner publishes to the community. Once it is published rest all users can make a copy and work on it.
so basically stories are not collaborative. may be Qlik will add this in future releases.