focus advice

Just a quick question. I have a d700 and typically use my 24-70 2.8 lens on it . I have noticed that if i do a group shot or even more than one subject one of the subjects are out of focus. Should i use the 9-21-51 points of focus or single focus and focus on the closest subject. I am shooting a wedding this weekend and I just am not sure where my error lies. Thanks in advance for any help.....
ergal55