For me, if I'm interested in someone that I know personally, then I've had good experience telling her openly about my feelings and situation. Worst case, she's flattered and we become/remain close friends. However, I don't have experience doing this with a relative stranger that doesn't know me already. In fact, I'm specifically interested in someone in my neighborhood, but I don't dare approach her because I don't know how she'll react, and I don't want to risk creating an issue in the neighborhood. (We still have to live there, after-all.) If we end up hanging out together and becoming friends, then I'll likely open up to her at that time, but otherwise I'm playing it safe because the stakes are too high.
Along that vein, I'd suggest talking with your GF about this, particularly if you haven't dated anyone else during your relationship with her, even if you don't pursue the other woman. That way, you can get a better understanding any concerns or needs she may have in order to feel comfortable with polyamory in general, and work together to address those needs prior to you ripping off the bandaid, so to speak.
I'd be prepared to go slow and focus on building a solid relationship with your GF first. If the two of you are fairly new together, then it may be difficult to build a solid foundation together while seeing other people as well, again depending very much on her specific needs.
If you're GF is ultimately comfortable with it, then I'd start by trying to become friends with the other woman, if possible. And if that's just not practical, but you feel the sparks, then I guess I'd invite her out after work to somewhere simple where you can talk, get to know each other a little, and open up to her.
I'm curious to know what advise everyone else has... I could use it myself as well.