As with Shorthand stories you can easily collaborate on a project with others in your team. Read more about collaboration.
Project collaborators can view and rearrange all stories in a project. However they can only edit stories they have edit access to (this is managed at the story level).
Conversely a colleague may have edit access on a story, but they are not a collaborator on the project. In this case the colleague can edit the story, and view the project (but not edit it).
If you want to edit Project-wide collaborators, you must do so from the Projects dashboard (i.e. ensure you first select Projects from the navigation bar on the left before editing collaborators on the Project). Otherwise, if you edit collaborators from the Stories dashboard it will only apply to an individual story, even if that story belongs to a Project.
Note that while you can safely work concurrently on a Project with teammates, at this point undo/redo functionality is not available on top-level Project settings, such as the organisation of the Project stories and the Project navigation settings.