I have recently bought skype premium in order to be able to share my screen during a group call. However, it was "grey" for a group call and "ok" for single person call.
Does it take some time for premium to kick in or is it not possible? Or does everyone in that group need premium or??
What version of Skype are you running? Click on Help > About Skype for the specific version number.
You need v5.8x to share screens on a group video call.
Update via this link: http://www.skype.com/go/getskype-full
I also suggest everyone else on the call check their specific version number (don't assume "I have the latest) and if not, they update as well.