Most people mistakenly think that because an image
looks unposed it was unposed, but that is seldom the actual case. If the example image looks really good chances are a lot of thought was put into it as to how to pose everyone yet make them look unposed.
Good images are thoughtfully created not mindlessly stumbled upon. If you think posing people is hard, wait until you try and not pose them and attempt to get a good image.
That is not to say it can't be done but it is hard. I suggest finding a place with good natural lighting outdoors (hopefully you live where it is warm year round) and then asking them to simply "do their own thing" then modify it as needed. Try and keep the faces close to the same height, which means seating the taller people (or even laying them on the ground) and standing the little ones.
Benji