Hello @mediacity, hopefully I can help a bit. First, if you shot the image in RAW format instead of jpg, working on an image in a program like photoshop will cause less damage to an image in the form of digital noise. Jpg's can still be worked on and I was able to make some adjustments to the jpg you posted.
What I did:
I opened the jpg in Adobe Photoshop's Camera RAW feature, but really, any decent photo editing program should be able to make the adjustments I made. I first straightened the image a bit so the door in the background was straight up and down. After the image was rotated, it needed to be cropped a touch so it looked like a rectangle again.
I lightened the shadow detail up quite a bit but then I darkened the darkest of the dark black areas of the photo so it brought back some contrast to the image and didn't look too dull or flat after lightening the shadows. I then lightened up the brightest white areas a bit to further add a little more contrast and a brighter look to the image. Those adjustments can change the color balance of the image, so next I adjusted the colors by adjusting the color temperature and tint to bring out a natural look.
I then added a touch of vignetting so the corners of the image would darken up a bit. This helped bring more focus onto the first three people in the shot.
Thanks for the fun @mediacity, I hope that helps!