Alright guys, there is an easier way to approach it and I say this with quite a few notches on my belt from experience. I never purposely intend to ruin anything, but I often find the highest quality women are usually taken. Unfortunately relationships often stale, and as human beings we have a moth-like instinct to be drawn to the shinier, more exciting series of interactions. Thus...your predicament.
First, establishing and building rapport with the girl is the most important aspect- even more so than usual. BUILD Comfort. The girl must think of you as that-guy-at-work that she's attracted to (she'll know it deep down and you'll see it, even if she won't openly admit it).
The key here is to build comfort with the girl, but just enough so that she doesn't think of you as a friend. Thus, you must mix attraction into it as well. There is a fine line here, and I break down the experience like this: Have fun with her as a friend, but constantly ping her in a flirty way. A great analogy I've heard before and the perfect one for this situation is to treat the girl as if you she were a bratty younger sister, or friend of your sister. You're older, and thus have a stronger frame, and you play around with her never quite taking her seriously. When doing this I never mention the boyfriend or talk about him. Why bother? There is no upside to doing so, and you'll only complicate your efforts.
Second, talk about things other than work. If you only talk about work with her, she'll envision you only as as a coworker. You want to be seen by her for who you are, and you want her to picture hanging out with you outside your respective venue. Talk talk talk talk, and eventually once you have enough flirtation and comfort, do something little with her to bring her out of the venue. Mention you have an errand to run while talking to her as you're both leaving work, and ask if she wants to run over with you. If she complies, you're getting closer.
The third step is to bring her out in a group setting (If she likes you enough, the boyfriend will mysteriously be busy that night. If he's there, don't fret). This step was particularly easy back in college, as you're both comfortable on campus and it was always a piece of cake meeting up with the girl later on at night in a group setting. For other locals, bring a group of people with you, preferably coworkers. Have everyone grab drinks out one night. You'll look like the cool social guy and build social proof with the approval of your peers. If you're under 21, hookah bars are the next best thing. Now- as I said before, don't fret if the boyfriend is there. She may just bring him to make herself feel better about being attracted to you. Once everyone is at the venue, don't be too aggressive with the game you've learned (you don't have to be). Use attraction material, but try hard to avoid canned lines and stories.
Once you've repeated these steps (culminating in everyone going out together) you'll start to notice heavier and heavier ioi's being thrown your way. You may even start getting IOI's from other female coworkers who hadn't given you the time of day before. Attraction is attractive. The girl will start to come up with reasons to hook up with you. Eventually she will, and it won't take nearly as long you might think. If she mentions her boyfriend, act disinterested and change the subject. You are not a psychiatrist and you'll speed things up MUCH quicker by not acting like one.