Do you already have the 50mm f1.8? That's a good portrait lens. Not as sharp or as bright as the much bigger, much more expensive "L" 50s, but still brighter than a f2.8 lens.
If you've got the 50mm, my advice is to take that to the party as your only lens. If you're going to spend money on something, spend it on a flash.
With a flash, you should be able to stop down, even at night. Try f/4 or f/8. That will boost sharpness. If you're concerned about weight, consider the Canon EL-100. It weighs 6.7oz without batteries. I'm guessing it takes either 2 or 4 AA batteries.