Photographing a group of people in full shade

[kitt2121]

I will be photographing a group from 6 to 20 people in full shade today. I have a Canon Rebel Digital and a Vivitar AF 728AFC flash. Any recommendations. Last time I took pics in the same place the faces seem to have a green or blue washed out look. I was not using the off camera flash though.
Any suggestions how this should be done, settings of the camera, AWB, Flash ??
Thank you.

[GB1]

You certainly need a fill flash, though exposure in this situation is tricky and I'm defintely not an expert with this. Strange that they came out greenish blue last time. The washed out look typically means that the flash output was too strong, or that you were too close (doubtful). Hope someone else can help more than I here

GB