I'm not sure how to approach a guy?

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There's this boy I find really attractive in my theatre class. We don't talk much, in fact he spends most of his time rehearsing lines with this girl who isn't afraid to be touchy (even though she's in a committed relationship) The really only interaction we've had is shaking hands during an exercise and being Facebook friends. I want to ask him if he wants to hang out, preferably with the girl and another mutual friend so that we can be friends before I even think of asking him out for coffee. We've just never talked. Should I talk to the girl so she can organize it and I can try talking to him then?

I'm decently tall and a little chubby (not too bad, its given me a nice rack and legs). My face is a bit plain and I have blonde hair that I usually pull up in a ponytail because its growing out from a pixie. He's tall and really skinny. I'm worried that my looks might put him off...