I think it largely depends on what the girl thinks of you. just think of what it would be like if it was reversed. there are some girls, that if they came up to you out of nowhere trumpets would play and some girls you wouldn't be interested in at all and some in between. plus girls are often approached by guys, so they can get pretty annoyed with it if they're not really interested in the guy.
you can avoid some trouble by making eye contact and shooting them a smile and see the reaction you get, if it's good go and talk to them, if not, then maybe it's not worth it to bother.
i think there are alot of things you can say, alot just depends on the situation, you'll want to introduce yourself at some point, and then just ask them some innocent questions, what they're up to or whatever and feel out the situation, if you can be funny that's good, if you feel they don't want to be bothered by you just be kind and say goodbye, if not just keep talking to them and then either ask if you can join them or rather ask them to join you or whatever, go and do something else, if it's that kind of situation, or if not, just ask them for a way to contact them so you can get to know them better. msn or something can sometimes be good for this.
i think whether it's creepy or not depends on your approach, your intentions, and the girl.