Group brainstorming is most effective when at least some of the participants are prepared in advance, and maybe even written down something, and then sent to everyone attending.<p>Most won't read, but some will. Thus, we will have people with different levels of prior knowledge and preparedness. Variability helps in the Wisdom of the Crowd.<p>But I've also heard that some facilitators are best at brainstorming without any prior knowledge or preparation, i.e. starting with a clean slate. I think that way it's easier to ask naive or "stupid" questions that might lead to some interesting solutions.