Whenever a document is shared with you, a notification is received. When you accept the document, you are given the option to either place the document in an existing project or in a new project.
Once you get a shared document or you share your own document, the icon in front of the document name changes to depict that it has been shared. Also, you could see the status in the Sharing column changes from None (unshared document) to denote number of users with whom it is shared.
I hope this answer is helpful. Do let us know if you have any other query.
Adobe Story Team