Well theres two ways you can go on this one:
Option A: Address ONE of the girls in the group, and after saying a few words to her, then address the rest of the group, maybe asking for the group's opinion, advice, etc. If you talk to only one of the girls, but totally ignore the rest, the rest of the girls will get b*tchy and may attempt to c*ckblock because they want their small share of attention (I know its ridiculous isnt it?). Ex: "Hey I need your opinion on my shirt, does it bring out my chest more or my arms more?"(Lets say she says "arms")..."Yeah thats what I figured, what do you girls all think? Chest or arms?" While all this is going on, be observant to see which of the girls is most interested, and then briefly single her out in a very brief convo, and then close it all out. Make sure you address the entire group when you close (obviously there is room for a number close).
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Option B: Address the group as a whole. This one can work well because it becomes easier to determine which of the girls is interested in you. The one that smiles at you and says something is the one you then focus on. If you're addressing the group, go with a question, comment, opinion etc that addresses the group as a whole. Ex: Hey do any of you have an extra pencil? (Now focus on the girl that responds to you, because she's the one that's interested)....Oh thank you, what's your name by the way?....Then just continue on with the conversation.
Let me add though, try to make it brief and that you will talk to her later, because her friends will get all pissed off and b*tchy with you, and will try to c*ckblock if you get in a long convo right in front of them or ask questions,comments, etc that require a long response. (They will view it as being rude). If going for a number, go for that first before you address the entire group to close out. Ex: "Yeah I havent seen you much lately, let me have your number so I can get to know you better....(smile) Okay see you girls later"