Good advice all around from Parts...but to elaborate on that, don't be the guy who makes little humorous (annoying) quips during the movie trying to score a laugh. You're not the main attraction here. Just sit quietly and cross your legs like a man, and later that night you'll be scoring in the projecter room.
Some younger girls really get a thrill out of riding "friends with crushes" for as much they are willing to shell out $$$$$$$$$$. This would leave you with bad feelings and no pie in the end.
If you take one piece of advice from this Wienerville support group let it be that one. Save your money until after a couple of dates.
Comic Book Guy: "Last night's 'Itchy & Scratchy' was, without a doubt, the worst episode ever. Rest assured that I was on the Internet within minutes, registering my disgust throughout the world."
Man, you're cool and she needs you a lot more than you need her. That's the impression that you need to be making. Subtle gestures- holding doors for her, looking her in the eyes, a quick touch on the back as she enters through the door that you are holding, listening to what she says will let her know that you are interested. But flowers on a group get-together will let her know that you are desperate. She needs to think that you have other options. Play it cool and have fun. Wish her a happy birthday in a sweet and quick manner and relax. On Monday, when she thanks you for inviting her, ask her if she's free next weekend and take her to dinner without the rest of the office there. Good luck.