You've probably got some suitable software on your computer already but, rather than trying to guess what you might have, I'll suggest downloading and installing Irfanview. (It's an excellent, free image-viewing and manipulation program, which deserves a place on everyone's computer):
http://www.irfanview.com/
Open a group photo in Irfanview. Click with your mouse at the top-left corner of the area you wish to select. Keep the button held down while you drag to the bottom-right corner. (You'll see your selected area framed by a rectangle on your screen). Go to Edit > Crop Selection. Then go to File > Save As, to save your cropped image with a new name.
Chris