Waiting until the mixer would work since you really don't know each other that well (or maybe you'll find out that you do...
).
Another thing you *could* do, if you REALLY want to see this girl, is give her a call and say, "Hey, I know we have that mixer tomorrow, but would you like to go grab some coffee first?" Then you can hang out with her one-on-one and you're guaranteed to at least see her for a little while. With a lot of mixers I've been to, there wasn't much time for one-on-one talking. If you do something fairly informal first - like the coffee - you have some time to talk and see if you'd want to take her out on an actual date. As a bonus, you won't have to try to track her down at the mixer, since depending on the activity, that might not be much of an option. If all goes well, you can either make a point to hang out with her that night, or, if you don't get to talk some more at the mixer, you've already found out if she's worth a second date.
Good luck and let us know what happens!!