I think about this sometimes.
First of all, approaching a girl amongst her friends already makes you way cool.
The Bible gives all sorts of tips like walking up and saying things like, "Excuse me, ladies, but can I borrow her for a second?" and point to the one you're interested in, but I find that would sound really dumb in situations outside of ones where gaming is expected (like clubs).
If you can think of a plausible reason to get her away from her friends, then go for it ("Hey, I want to show you something I thought you'd like," but then you have to show her something, and it can't be your schlong). Otherwise, just walk up and interact with the group, but make sure you 1.) maintain your presence as a man (don't be around when they're talking about their periods or something) and 2.) project non-verbally which one you're really interested in. Look for eye contact often, especially when she isn't talking. Talk more to her than to the group. See if you can discreetly get her number, and only her number.
I hate it when people giving advice don't get specific enough, so just ask if this is too vague.
