This is a pair exercise.
One player says to the other "Let's go on an adventure".
From that point onwards each player must start each sentence with "Yes and...".
They continue the story alternating lines.
Let's go on an adventure...
Yes and we can both wear matching jumpers so that people know we're on the same adventure.
Yes and each jumper could be identical except mine will have your face on it, and yours will have my face on it.
Yes and whenever anyone notices we can point to each other and smile.
You can also get the group to tell a new story with the following line openings:
Yes but - Agree with the other players idea but then introduce an idea of your own with no relation to the first idea.
No and - You dismiss the other persons idea outright and must essentially start the scene again.
Ask the participants - "How would you characterise that conversation?"
"Yes and" gives you characters who like each other. It's energetic and fun and you never run out of ideas.
The two negative variations make people hate each other, close down communication and keep making the scene start from scratch.