There's no magic opener that's going to get the girls interested. You should be more concerned that your mates know how to follow up the inevitable replies of "No"/"Yes"/"I think so" etc.. as in my experience guys who lack social skills will just mutter some quiet follow up and then hide behind their bottle of bear.
Tell them to comment on what she's wearing (clothing/jewellery/shoes etc..) and then do the old 'Question-statement-question' to get her interested.