Yes it is. Two ways;
From iPhone/iPad/iPod etc, you will need to have the older version of the grooveshark app. (The one that was released in App Store, no pulled). The newest version of Grooveshark found in Cydia is much better, but it does not have the Airplay function for some reason. Will hopefully come soon, though.
From Grooveshark for mac, you'll have to download an application called Airfoil. Airfoil can stream any audio from your mac to airplay-devices.