Your team can work in the same story at the same time. You do not need to do anything to enable it — you can simply open a story someone else is working on, and work on it at the same time.
To see who else is currently editing the story, look for the highlighted user avatars in the header, as seen below:
You'll also see each editor's avatar within the section they're editing.
Note: When multiple users are editing the same story, there can sometimes be unexpected results. While rare, this can particularly happen when changing one section type to another, or when multiple undos and redos happen concurrently on the same element. We suggest keeping an eye out for anything strange or unwanted before publishing.