I agree, the severe underexposure is a big problem. If that is already edited to brighten them up, the it must have been really bad to start with.
Lighting a group can be tricky. You need to remember that light falls of at an inverse square to the distance. So if the light is 8 feet to the first person, and the last person is another 8 feet behind that...the last person (being twice as far from the light) is only getting 1/4 of the light.
This is why, when you are lighting a group, it's important to have then relatively the same distance to the light, or as close as you can get them. Or, use more than one light.
I know what you were going for with #1, but that shot does require more than one light, or better ambient light.
But either way, the exposure needs to be brighter.