Twine 2 doesn't have the ability to share your story directly with someone else right now. For now, you'll need to export your story to a file using either the Publish to File option in the story menu (to export a single story), or the Archive button on the story list (for a set of stories). You can then give your collaborators this file to import in turn.
Keep in mind that when a file is imported, Twine 2 doesn't check to see if there's already a story with that same name, so you may see multiple versions of the same story in your list. The process of fixing this is being tracked here, but in the meantime, check the edit date of your stories in the list to determine which is which.