Collaborating on Stories
Twine doesn't offer a way for multiple people to edit a story at once. The most basic way to collaborate is to export a story and have your collaborators import it to work with it themselves.
Another possibility is to use source control to share your stories, but this means learning how to use software like Git. There are separate applications that can make working with these systems easier that might be worth investigating if you're not already familiar with them.
If you're using app Twine, using cloud storage services like Dropbox or Google Drive to sync your story library is not recommended. It is very easy for these services to get confused as to the state of your story files--they often try to merge changes from different sources together on your behalf--and cause your stories to become corrupted.