According to the manual for your flash, if you use the flash in "Auto" mode, the distance opposite your f/stop on the setting dial is the maximum distance that the flash will illuminate, and the minimum distance is about 10% of that.
So the f/8 auto range is between 1.85 feet and 18.5 feet.
A couple of hints:
1) Get up a bit higher (on a chair?), and shoot down at the group;
2) Have the flash start even higher;
3) Bounce the light off something reflective, and a bit larger than the flash head;
4) Make sure the sensor is pointing at the people at the centre - same for the smaller, secondary flash;
5) If you can, have the people in rows - on risers if possible.
With regard to #1 - watch out for low-cut dresses.