Dan M , May 15, 2010; 01:36 p.m.
Many cameras will not synch with flash at speeds greater than 1/200. Check to see if yours has a high-speed synch option. That might buy you a little bit more. Also, turn down the ISO as far as you can. However, once you are down to your minimum ISO and max speed (presumably, 1/200), there is no way to let in less light other than closing down the aperture.
You can get plenty of blur at apertures somewhat smaller than 2.8 if the background is a bit of a distance behind the people.