Treat it like a set in a bar, only split the approach over time.
By this I mean enter the group do all the usual stuff like a canned opener, time restraint, negging the target ect and then do some DHV stuff to the whole group. Then say you gotta get back to yer friends and leave.
Now the next time you see her you should have already built up some attraction and her friends will have seen what a cool guy you are thus giving you social proof making isolation that much easier. Her friends will probably end up helping you if you just go up to her with a reason to remove her from the group just by saying something like "Come with me a sec, I need your help with something". If your up for spontanious adventure you could just run up, grab her hand and pull her along with you, though remember you need something to show her or take her to or she may start feeling uncomfortable.
Hope this helps, feel free to correct me anywhere if you disagree with what I've said.