Nadine Ohara - SF Bay Area/CA , Jun 10, 2008; 02:30 a.m.
You have a very long extension cord, right?
The best is sun on the subjects' backs and even shade on their faces and fronts. Expose for the bright sun, fill the shaded parts with flash. If you can't get most of each person's face in even shade (no sun hitting bits of noses, cheekbones, etc.), then use the sun as a key light and the flash as fill, but you will have squinting.
If you only have one light (why not use the 430EX as on camera fill?), it is best just immediately to the left or right of the camera, unless you have small groups, couples, and individuals, where you can push the light further away from the camera (assuming you have on camera fill)--the typical key/fill arrangement, between 30-45 degrees away from the camera..
If you use modifiers, hope for a windless day or be prepared to sandbag your stand and light heavily (that may not even work) or have an assistant holding the stand and light in place. I would not even use a modifier, since sunlight is already hard light and at the distances you need to use the light for groups, a modifier isn't going to soften much anyway. If you do use one, I would go for a 60" umbrella for ease of set up, or an umbrella box, but larger the better.