There are various ways you can collaborate using Fade In.
The easiest and most robust way is to use Fade In Access, Fade In's optional service for cloud storage, online editing, and more, which allows you to share documents with other Fade In users, to be worked on either separately or at the same time.
Other than Fade In Access, the old tried-and-true method is to work independently on separate copies of the screenplay and have one collaborator be responsible for combining the work. The Document > Compare to Previous function to find changes between versions can be very useful for this.
You can also save a common copy in a shared folder on a cloud service, such as Dropbox, Google Drive, or OneDrive and give each collaborator access to that folder or copy. (Although this has the disadvantage that changes made at the same time to the same copy may conflict.)
Syncing using a cloud storage service
A third method is for one or more collaborators to work together in real time using Fade In's collaboration function. In that case, one user starts a collaboration session and invites one or more other users to join. Collaborators can then work independently on their own locally copy, and any changes they make are propagated to the other participants in the collaboration session.
(Note: Legacy realtime collaboration has been deprecated with the introduction of Fade In Access, which provides for much better collaboration functionality. You can still use legacy realtime collaboration, but it must be turned on under Preferences > User Interface.)
How does realtime collaboration work?
Finally, depending on the situation, some collaborators have used Google Docs as a method of collaborating, writing in Fountain format and importing into Fade In, which can readily open Fountain files.