Asking help using Nikon SB600 as fill flash?

I'm going to be taking outdoors group shots soon, so I bought the SB600 I've been wanting for a while for the occasion. The complexity compared to the cheaper flashes I've used before is a little daunting.

I use my Nikon D90 in "M". For fill-flash of people's faces (not the background) I think I should use the flash in "TTL" and not "TTL BL".Is this correct?

And I'm not sure when to use the diffuser. Or not use it. Would I use it when outside as a fill flash?

Thanks, I'm not going to get much practice time before the shots have to be taken! Peace!

First off, for a group shot from a distance in otherwise bright sunlight it may not be powerful enough.

The difference between TTL and TTL-BL is that the former basically uses matrix metering for the flash exposure, while the latter uses center weighted metering. If your subject is in the center, use TTL-BL. Both should work reasonably good.

Anyway, use manual mode and adjust ISO and aperture for a proper exposure while metering the background. Keep the shutter speed at 1/200s (i.e. The maximum flash sync speed) to get the most power relative to the ambient light out of your flash.

When you then point the camera towards the group, the light meter will probably show a slight underexposure. That's the amount of light that the flash will contribute then, to boost the brightness of the group to a proper exposure.

A diffuser may be a good idea but it may further diminish the power of your flash.It'll have a really hard time to match up with the sunlight already without a diffuser.