I was asked to take a group photo at our work for their website. I selected a few spots in our building which looked promising. However, I was not impressed by the results. It shows that I still have a lot to learn! I'd appreciate your feedback: how can I improve these photos?
1. This was taken with Sigma 50-150 f/2.8 in a large atrium with diffused natural light. It looks a bit bland. I don't want to increase saturation, though. How to make it more lively?
2. Another Sigma 50-150 f/2.8 in the same atrium. Similar bland effect.
3. The lens was Sigma 17-50 f/2.8. The atrium is in the background, I used a hot-shoe mounted flash with a diffuser to fill in. Perhaps not enough? Surprisingly, this image is quite noisy. What you see here is a processed version with increased contrast and removed noise, but the original is quite noise, despite using ISO200. Underneath, there is a 100% non-modified (i.e. ACR conversion with default parameters) of the face. I guess it was underexposed and when I brightened it up and increased contrast, the noise came out of darkness.

4. We had huge windows in our offices with nice views, so my idea was to sit them against one of them and use flashes to get some light. This turned out to be very tricky, as it was difficult to set up two umbrellas without getting reflections the glass. Below, a not entirely successful attempt. What happened, and I noticed this only in the computer, couldn't really see it on my camera screen, is that because the umbrellas had to be positioned quite deep on the left and right (to avoid reflections), the front row of people is not illuminated correctly (side lights are too harsh). The second row looks perhaps better. In the hindsight, I'd probably need a third flash underneath the camera to fill the shadows. It could be low enough to avoid reflections.
Anyway, my impression is that pictures 1-3 look dull and I'm not quite sure how to improve them. Any help?
1. This was taken with Sigma 50-150 f/2.8 in a large atrium with diffused natural light. It looks a bit bland. I don't want to increase saturation, though. How to make it more lively?
2. Another Sigma 50-150 f/2.8 in the same atrium. Similar bland effect.
3. The lens was Sigma 17-50 f/2.8. The atrium is in the background, I used a hot-shoe mounted flash with a diffuser to fill in. Perhaps not enough? Surprisingly, this image is quite noisy. What you see here is a processed version with increased contrast and removed noise, but the original is quite noise, despite using ISO200. Underneath, there is a 100% non-modified (i.e. ACR conversion with default parameters) of the face. I guess it was underexposed and when I brightened it up and increased contrast, the noise came out of darkness.

4. We had huge windows in our offices with nice views, so my idea was to sit them against one of them and use flashes to get some light. This turned out to be very tricky, as it was difficult to set up two umbrellas without getting reflections the glass. Below, a not entirely successful attempt. What happened, and I noticed this only in the computer, couldn't really see it on my camera screen, is that because the umbrellas had to be positioned quite deep on the left and right (to avoid reflections), the front row of people is not illuminated correctly (side lights are too harsh). The second row looks perhaps better. In the hindsight, I'd probably need a third flash underneath the camera to fill the shadows. It could be low enough to avoid reflections.
Anyway, my impression is that pictures 1-3 look dull and I'm not quite sure how to improve them. Any help?