When we have our family reunion, which usually results on about 15 kids in the pool, I increase FC level to the top end of the CYA/FC range with chlorine. Usually do this about 2 or 3 hours before I think the pool is going to be used.
Will also measure the level after everyone leaves and use more chlorine to bring back to, or just above, the recommended target.
If you have it I would say use the chlorine, and if you can add before you have the party.
I have found using liquid chlorine the fastest way to boost levels after a party. I tried using SWG one time, but the level wasn't increasing fast enough for me to feel comfortable with it. 1 or 2 cups of 10% chlorine and that was all it took to boost it quickly and make the water clear again. So now I keep a jug handy for the times when we have a lot of people going swimming. @-MacGyver- 's idea of adding before is a good idea I might try as usually after a few hours with a lot of people swimming, the pool starts having a haze.