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?
Good point, the about page is out of date now. We removed it in the current version because with the URL anyone can view and edit the diagram and we had commented about access and permissions. Also, a large number of our users use Google Drive for storage and the collaboration didn't follow the Drive permissions.
Currently, we're rebuilding this functionality to comply with the Drive file permissions, but the current timescale for this is May 2013. If you want to use sharing now, you can use the previously released version, from the menu select File → Share and give that URL to whomever with which you wish to share the diagram (the parameter is too complex to be guessed).
So you don't get stuck with an old version we'll add a URL parameter to enable it in the next version (the menu option is just commented out at the moment).