Met a girl, she has a boyfriend, so what to do?
It’s highly probable that she’ll view this as just creepy. She’s already given you the brush-off. I’d advise only meeting her under professional circumstances. If sparks fly at that point, great… but from what I can tell, women are sick to death of guys who say “I know you’re taken but I’d like to be friends” and then do a poor job of hiding the crush.
Don’t do it. You won’t have any regrets if you leave her alone. Better to keep your mouth shut than to have cringefests of regret later when you have come to your senses. You have already taken the step to ask her out. She declined. She is not interested. Of course you can say hello if seated next to her in a professional setting or something, but other than that–let it alone. You will find another girl that is available. She will be just as smart, beautiful, and easy to talk to. It’s a heartache crushing on someone that is taken, but it will soon pass.