I'd like to use Draw.io in real time collaboration with a colleague—the About page says it can be done—but I can't see any options. How can I do it?
For how to share a draw.io diagram with another user generally, see the FAQ entry on the subject.
Repeating the current content:
You can share a draw.io diagram stored on one of the cloud storage options with another user using the built-in sharing functionality of that platform, https://drive.google.com in the case of Google Drive, https://www.dropbox.com/home in the case of Dropbox and https://onedrive.live.com in the case of OneDrive.
Google Drive also allows the option to share within the application itself, using the blue "share" button top right in the app.
Note that only Google Drive supports real-time collaboration between users, Dropbox and OneDrive are last write wins.
If you are not using a cloud storage option and are saving the diagrams locally, you must manage your diagrams yourself, we do not have copies of the diagrams and cannot offer sharing functionality.