Notice that this image is far from perfect, but it is acceptable. It clearly shows some of the standard problems you usually run into with a large group and small flashes.
I used my camera in Manual mode and the flashes in TTL mode. I used the pop-up flash as the Commander with a Nikon SG-3IR filter in front of it. The SB800 only responds to IR light that enters through the small round red window, and since the the pop-up Commander puts out both visible and IR, the SG-3IR is useful to supress the visible portion of the light to help keep people from blinking.
In a large room like this, where there are no walls or ceiling from which the preflash pulses can reflect, the SB800 flashes in umbrellas must have their round red IR windows facing the Commander, or the preflash pulses from the Commander will not control or fire them.
As you may be able to tell, the ambient light was strong, and it was some weird mercury vapor color so there was no way to match the flash to it. Therefore, there is a slight color shift between the front row and the back row indicating that the front row was lit mainly by the flash, but the farther away from the flash you get the more the ambient had an effect.
Holding focus from the front row to the back row is always a challenge. You address this by creating as much depth of field as possible by using as small an f/ stop as possible. However, the smaller the f/ stop you use, the more power is needed from the flashes to provide enough light.
You also want to have the rows stand as close together as possible. Notice how closely the rows are standing to each other in this image.
I used f/5.6 on this shot, because that was the smallest aperture I could use and still get adequate lighting from my flashes. I set the camera in Continuous Servo AF with the switch on the front of the camera, CSM a1 to FPS Rate (ie, Release Priority), and CSM a6 AF-ON Only (ie to focus only when the AF-ON button is pressed). This allows me to aim at the second row, push the AF-ON button to focus (or use manual focus if I want to), and then recompose to place the group in the frame as I want it, and then push the shutter button to take the shot, and the green focus light doesn't have to be on for the shutter to release. This shifts the focus toward the middle of the group so depth of field is optimum for getting everyone sharp. F/5.6 is barely adequate for holding focus across a group this large. F/8 would have been more desireable if I were using say three or four SB800's.
I put my two SB800s in reflecting umbrellas on my ten foot stands extended as high as they would go, and angled them down so the light would come from above the group as much as possible which minimizes the fall-off of light from front to back.
Also, for a large group, you must place the umbrellas as close as possible to the group to minimize the Square Law light fall-off. In this case, I placed the umbrellas about three feet on either side of the camera and moved as close to the group as possible. This meant I needed to use the 17mm wide angle position on my lens.
I also had to leave some room at both sides of the frame for cropping. If you fill the frame from edge to edge, you will be able to print an 8x12 print, but not the standard 8x10. I have never been able to find a low cost 8x12 frame anywhere, but 8x1o frames can be bought at Walmart for $6.00. When I shoot a group, I simply look at the frame as though it is an 8x12 print and remember to leave one inch on each end.
I used a fairly slow 1/60th second shutter to allow the background to show.
Then there is the reflectivity issue. In this shot the group was mostly dressed in all-black, so the problem I had was to get the remote flashes to fire at the correct power. Black does not reflect light very well, so the flash metering system thinks it needs to increase the flash power high enough to create the reflection. This will totally blow out the faces. In this case, I decreased each remote channel on the Commander by -1.3 ev, and that corrected this problem.
Once I got the flash power where I wanted it, I locked it in with the FV Lock button. This button fires the preflash sequence and locks in the resulting power as determined by the flash metering system. This step is very important if you will be taking several group shots and you want them all to look the same.
One more problem with shooting groups is how to get everyone's eyes open. I tell everyone to blink quickly for a few seconds, while I am counting to three, and then I say 'Hold your eyes open' just before pushing the shutter. That works pretty well, but I also take at least three shots of each pose, so if there is a person with eyes closed, I can copy just the eyes in photoshop from a good image and paste them onto the final group image.
Additional Information On Shooting the Group Shot Above:
When you use SB800s in remote mode the red AF-Assist lamp in the SB800 doesn't come on, so then the choice of AF-S versus AF-C is only based on focus performance in ambient light.In this case, I was using the pop-up flash as a commander, and for group shots I usually keep the white AF Assist lamp in the camera turned Off with CSM a9, because it causes some people in the group to squint.
Obviously there has to be enough ambient light for the focus without any AF assist lamp. If there isn't enough light, then I normally go back to an SB800 mounted on the camera (or on a bracket) in commander mode, and switch to AF-S so the red AF Assist Lamp will come on.
However, assuming enough ambient for focus, in addition to AF-C, I have set the focus to use only the AF-ON button, and the priority on FPS (Release Priority). This way pushing the shutter button no longer has any effect on focus.
The main thing I am trying to achieve is accurate focus on a specific point somewhere in the middle of the group. I have found that with AF-C mode it is simple to hold down the AF-ON button and move the camera aim around and observe various points within the group pop into focus. Then, when I get the point I want, I release the AF-ON button and the camera stays focused at that spot while I recompose to place the group within the frame where I want it. Then the focus stays fixed on that point for a series of shots. I also watch the green focus lamp in the viewfinder to make sure the camera thinks it is in focus when I think it is.
I could use AF-S mode in Release Priority too, but then to get the focus on exactly the point I want requires pushing and releasing the AF-ON button repeatedly, because in AF-S mode, once it achieves what it thinks is 'focus' it stops trying, and it usually doesn't end up focused on exactly what I want it on. I like the focus to keep 'trying' until it gets to what I want.
Another way to handle focus is to use the AF to do the initial focus and switch to Manual Focus once focus is achieved. Then focus remains fixed for the rest of the shoot.This is actually the way I do it most often when using a tripod.
And for this group shot, I used ISO 400 and WB on Flash. Sometimes I use Daylight WB when shooting flash, which will make the image slightly cooler. But I never use a custom WB when using flash, unless I use gels to match the flash to the ambient. Then, I use PRE to set a custom WB that matches the ambient.