I took photos of a family of 4 a few weeks ago, and after all was said and done, I brought them up on the computer to view and edit and realized I only had a few shots where their faces were in total focus together!! What did I do?? I've taken successful group photos before, so why was this one different? And then in my research I discovered my big no no. well, 2 big no no's.
1) I used the wrong lens. Really a 50 mm is not the best when trying to squeeze more than 2 people in a photo. I ended up cutting off limbs or found myself a mile away from my subjects to squeeze them all in! I should have used my wide angle lens. In fact the previous families I photographed I used my 18-105mm. I tend to keep my 50mm on my camera because of the sharper quality of the photos and the aperature can be opened up to 1.8 allowing for so much more light to enter in low light conditions. This leads to my second no no.
2) If you shoot w/ the aperature wide open, you likely won't have enough depth of field to keep everyone in focus. This was exactly what I did. I opened it up to 1.8 and clicked away. This left mom and dad (standing right behind their kids) in focus, while their kids were very much out of focus.
So, what i've learned so far w/ out getting too technical (because I don't understand the technical stuff anyway) If you take a group photo of say 4 your aperature should not be set smaller than 4. At least that would be the place to start. Giving you a wider depth of field to keep everybody in focus. And next time i'll leave the 50mm in the bag and bring it out for portraits and such.
My example of a photo gone wrong - the photo is small but you can probably tell the kids are pretty fuzzy.

50mm Iso 320 1/2000s f/1.8 -2.0EV
now here is an example of what I did right or started to anyway.
(these last few have been edited in photoshop)

50mm Iso under 200, 1/250s f/3.5
so I still used the 50mm, but overall the photo looks to be in focus.

50mm ISO <200, 1/640s f/3.5>

45mm ISO < 200, 1/125s f/4.8 >
My disclaimer: I'm no pro, only learning as I go!!