One of the main rules of photography (as always rules are meant to be broken) is that things (and people) should come in odd numbers. 1,3,5 etc. This is particularly true of objects and items but does apply to people as well. Groups of odd numbers of people always look better than even numbers, the only possible exception here is 2 people as they are so close to each other they become 1 item. Take a look at the following images and see what you think about the odd number rule.

2 people in a photograph breaks the rule of odd numbers but works here because they become 1

3 people here looks better than the next image where there are 4 people

4 people here not as good as with an odd number