I'm a female, and was clubbing with female mates, when 1 of them (while flirting with a guy at the bar) turned to smile at me & brushed her index finger under my chin. I'm straight & as far as I know so is she, was she just being affectionate or did it mean something more?

It might have meant something more, who knows except her... I personally wouldn't read into it too much, if she was trying to come on to you and you were out in a club drinking, I think she would have been trying to get close to you and dance with you as well, a simple touch of the chin is too subtle to be anything more than just playing around.

Maybe she wanted to do it in front of the guy she was talking to, so that he got turned on, you never know... Or it could just be a kind of flirty affection.
But if you think there's something there that you want to explore, try doing something similar back to her and see how she reacts.

My first girl on girl kiss was in a club too, although we partly did it to get guys attentions, but secretly I enjoyed it sexually too. That's before I knew I was into guys and girls though, but now I am out as bi and very proud to be so!