If you receive the message "Sharing is unavailable at this time" persistently when trying to share a diagram with another Google account, the issue is most likely a cookie on the google.com domain has gotten itself into a state where it's not able to refresh. Try clearing cookies, or, if you know how to, clear just google.com domain cookies and try again.

Note that you can also go to https://drive.google.com, find the draw.io file, right-click it and select "share". This will bring up the same sharing dialog.