The rule of thumb for groups is use the longest lens (or focal length) you are able to get the whole group in the frame. The reason is perspective distortion. With a wide lens (and therefore working closer) you have tendency to get subtle perspective distortion. The subjects in the front row appear larger than the people on in the second row and worse if you have 3 rows. Sometimes a wide angle is required, but the longer the lens the better.
Thanks! This makes a lot if sense.