Usually in a group setting you have the outspoken people that do not normally have the best ideas and the others that might have better ideas that are afraid to speak up. It can easily get into a rut that surpresses creative thinking.
What I see in group brainstorming is the same obnoxious people trying to push their ideas on others. I'm always the one that speaks out against the pushy people, my last boss said "don't hold back, tell us what you "really" think".
He appointed me to the company feedback panel for new product deployment and I was the one to escort our Executive Management when they were in town so I could share my ideas on an executive level. So, don't be afraid to knock the pushy people down.