The opener isn't even important, it's what you say after she's responded that counts. Think of it like this.. whatever a girl says is a conversation hook that you can use as a fun/ witty response. Pick out the main things in her response to hook to.

For example: "I was at the mall shopping and I bought this awesome skirt" (couldn't think of anything else)

What you can do is take all of her hooks and start a thread of conversation.
"Oh yea, what mall do you go to?"
"Oh no, don't tell me your one of those shopaholic Sex and the City types"

If the group is laughing and you have nothing I would just ask how everyone in the group knows each other.