It also works exactly the same on the desktop; just use the ‘@‘ symbol before names to mention friends or coworkers, and the ‘#’ to reference known issues.
Oh yeah — you can also use ‘:’ to add emoji to your commits and pull requests. Yay!

The new features are available for both OS X and Windows, but you’ll probably need to manually check for updates in the settings menu on GitHub Desktop and relaunch the app to get things rolling.