[orwin]

If they are performing the songs live, they do not need a license in most european countries if it isn't recorded (not sure about UK though, i've heard its a bit weird up there for musicians). If they're just playing CDs, they might want to get a license, even with this "trick" that some bad DJs use.

BTW: good/known scenes have a license to play, not just perform, music live, so even that would be weird.