Indoor conference photo shoot backlit problem with my Nikon?

I was at a 30' x 40' conference room (incandescent light) shooting the panelists, however the big open window behind some panelists allowed too much light so they come out all dark, except the window is bight and overexposed. I'm using Nikon D90 with 18-200mm 3.5, with SB800. I tried different ISOs and tilted my SB800 to different directions but the panelists still came out dark and grainy.

What did I do wrong in settings? What are the correct settings?

Your professional advice is greatly appreciated.

That's what exposure compensation is for.

This high contrast shot is one of the hardest photos to achieve, and it does illustrate the fact that cameras do not have as much dynamic range as your eyes. You need to use a technique called Fill-Flash.

If you set the exposure for the window, the subjects in front of it will be too dark. If you expose for the people, the window will be too bright. Exposure compensation does not work as it only gives you the choice of one or the other (background too bright or foreground too dark).

To get the correct shot, you have to augment the natural lighting with fill-in lighting from your flash. This is one time when you would use the flash in the daytime.

To do this kind of shot, you should set the camera's exposure to properly expose the window. This will result in under-exposing the people. However, you then set the flash to properly expose the people.

This works because the flash is short distance, and will not affect the background lighting, only foreground.

You may have to put both the camera and flash in manual exposure, but with some experimentation, you should be able to get it correct.

Here is an example with my D90, 18-200, and SB700.

This first photo was photographed exposing for the window. This is probably what your photo came out like:

The second photo was photographed exposing for the foreground.

Neither photo is good.

To correct this, I used my SB700 and set both the flash and camera's exposure to manual, and set the camera to meter the background (as in the first photo). The flash then was set to properly expose the foreground (as in the second photo).

This is the result:

The reason it works is the flash is not powerful enough to change the background lighting - only the foreground. And some tweaking of the settings might need to be done to get it just right.

So, in summary:

1.set the camera to expose the background.
2.set the flash to expose the foreground.

Here is a webpage showing the photos side by side:

http://www.althephoto.com/concepts/composition1.php

Hope this helps.