If there was a girl I had interest in, I would look at her with intense eyes and then ask her out right on the spot.
I would leave a great impact on her by saying one thing, "You're the first." And then she would have ask, "First what?" But I would just walk away silently while making a V-sign with my hands.
I really suck at these things, so I would probably just be like, "Hey, Seungri.. I like her right there" or tell the other members who I like instead of going after her.
For me, I would slip my phone in her back pocket as I walk by.
That means you would have to touch her butt.
......then I'll slip it in her front pocket..;;