There are lots of factors to consider in order to get a great group photo. Here are some key points:
Location. Is there a good space availble, which looks good and where everyone can fit comfortably? If the group is large then they should either be arranged on multiple levels, ie on a slope, stairs, etc. If this isn't possible then is there a way that the photographer can look down at the group from above?
Lighting. When shooting indoors I will almost always use flashes, so lighting isn't a problem, however when shooting outdoors direct sunlight should be avoided as it can cause uneven lighting across the group, harsh shadows and squinting. The only way to easily include sunlight is from behind, but carefully as when it can give people red ears, due to the sun shining through the blood in the ears.
Posing. When posing groups it can be a good idea to incorporate furniture, architecture, etc for people to sit, recline or lean on. This will give a more natural feel to the shot and create a more dynamic composition. Posing each person individually will influence the overall composition, which should be balanced visually. Also groups should be arranged in such a way that peoples heights balance out, men and women should be interspersed, and colours of clothing should also be taken into account.
Atmosphere. Body language and facial expressions are important, so it's important the photographer brings out the best in a group to create a natural and relaxed atmosphere.