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How to talk to a girl in a group
Evaluate the group dynamic.
Not every group is going to be appropriate to approach. Is this a group of girls commiserating over a group member's break up? Are they talking about work or is a family member sick? Listen in without seeming obvious for cues.
If it is a serious topic you're best not to talk to her. If it is light-hearted, then you're probably safe to proceed.