I'm very informal but I have to say that 'guys' makes me cringe, and as it's gender specific I would imagine that the fairer sex would find it particularly jarring. 'Folks' is better but too American,and if they want to keep the informality I would suggest that perhaps "hello everybody" might be used.
Kulula.com call everybody "Superheroes, boys and girls" which I hate, and ultimately I fear that the classic 'Ladies and Gentlemen' or 'ladies and gents' is the best option.