This activity can be conducted with two or more people - the more the merrier! Everybody begins free writing. As you come across a word you think is interesting in your story, shout it out. Now the others have to incorporate that word into their stories as well. You can go around a circle shouting words or alternate between two people, or just let anybody shout out at random.
This exercise is a great test of your adaptability at times, when your writing mates shout out something completely different from what you have going on in your story. But that's what makes it fun! See how different your stories are, even though they incorporate the same words throughout.
Here are a couple of examples of this exercise in action:
Be a Tree