This can be done with a large group. Ask for a volunteer to write all ideas on a blackboard or flip chart where members can see and hear ideas. Feel welcome to add ideas quickly. A key word from every idea will be noted. No judgment can be made about an idea.
Creativity is good.
It's O.K. to expand an idea that's already been mentioned. The leader's role is to help capture key words from every idea that is mentioned and invite comments from members who have not spoken. When no more ideas are being added, the leader should stop the brainstorming session. It is helpful to take a break after brainstorming to allow members to look at the list and discuss some of the ideas. Then the group should establish criteria for selecting the best ideas.