Group photo with a 50mm 1.8G lens?

I own a Nikon D3100 with a 50mm 1.8G lens, how can i take sharp group photos focusing on everyone?

the few times I've tried, the photo usually comes out focusing on one or two people but blur on the rest of the group. How can I solve this? Go further away?

Set the aperture to between F8 or F11.

Read up on depth of field.
http://www.dofmaster.com/dofjs.html

F/1.8 is quite shallow, you need to reduce the aperture and check the DOF using the cameras preview button check what is in focus and what is not. You'll probably find you need to drop the aperture to somewhere between f/4 and f/8 depending on the size of the group. If you are taking this photograph indoors it'll probably require some type of additional lighting.

Bump the aperture up. Lower aperture will give you more blur.

Learn about depth of field!

Shoot with the lens stopped down (eg f11 or f16) and focus on about one third into the picture (hyperfocal distance).

With a smaller aperture your shutter speed will be longer, so you may need a tripod, depending on ambient light levels.