There is a Policeman's Ball at the Old School Hall near the Callahan Bridge. Luigi wants you to take some of his girls there to make some money.
Making Some Cash
Firstly you should be aware that you have 5 minutes to get at least four girls to the ball. Now that's out of the way, I suggest you use a four door car, at least. You could also use a Coach which can hold a limitless amount of people, however it lacks in speed, so I'd say go for something like a Rumpo or Sentinel, even a Taxi will work well. All you've really got to do is drive to the locations on the radar and pick up the girls, then drive them to the Ball near the Callahan Bridge. You need at least four, but you'll get a bigger bonus the more you take. I suggest starting with the girls in Harwood, then Saint Marks, then the ones around China Town. Don't kill any of them and go as fast as you can and you should be to deliver all 8 with close to half a minute left.
Mission Passed!
$4000
Additional Notes
You will receive a $500 bonus for each additional girl you take to the ball after the required four. This means you can earn up to $2000 in bonuses for taking all 8.