This is a group writing activity suggested by comedian Rich Williams.
Take a hat, or some sort of container and a sheet of paper. Cut the paper up into strips long enough that someone could write at least a sentence on. Divvy up the pieces of cut paper to your group. Throw out a subject matter to the group, then mark the clock and for the next five minutes, write as many ideas or premises related to that subject matter in five minutes. At the end of the five minutes, put the ideas/premises into the container, then have the group pick a slip from the hat at random.
Once the slip is selected, write or riff for five more minutes regarding whatever is on the slip. If writing, expand on the idea, make it your own. If working out audibly, see where the idea/premise takes you in relation to your own life/experiences.