Hi,
The people in your photo are underexposed -- as was noted. You did the right thing in the way you measured the exposure and set up the lights.
However you let the camera and it's image processor take over from there and process the photo.You have a 7D (nice camera) but the DIGIC IV image processor in that camera sucks. I know, I have one too.
You're next big step is to start shooting RAW and bypassing the camera's software. Canon would make it work if they could; so would Nikon, Olympus, etc.. -- they can't.
So when the camera processed your exposure it included the background in its calculations and compressed the exposure for your subjects. Here's a file:
I inset the histogram for your original in the upper corner with the original. It tells you what's going on here. The point I marked as the problem is where the background begins. In order to have room for that background the camera pushed everything else to the left.
If you have a section of a photo that is underexposed it will also be flat (lacking contrast) and have lower color saturation. An exposure correction alone will not address the contrast and saturation errors. A Levels adjustment is indicated. I corrected the tone response error in Photoshop using Levels. This simultaneously raised the contrast and saturation. To do this I had to use a mask to protect the shirt.
Post processing can remedy much of what your camera's image processor will screw up -- but the real answer is to bypass it.
You're almost there. You set your lights up well and metered correctly. You got a great camera; flip that switch to RAW.
Take Care,
Joe