The Musical Impov Prompt Generator can be used in all kinds of music playshops and Jams. Scroll down for more info.
I've used the cards and then the prompt generator with from two to five person jams. I was getting all excited about using them with big groups when Delta hit. Regardless of how many people there are, the idea is that you click the "New Prompt" button and interpret the directions in any way you like. If you get tired of your prompt, you can click again for a new one. And if you don't like your prompt, you can just skip it and try another one. If you'd like to see the physical cards in action click here.