I'm okay brainstorming on my own, but I prefer brainstorming with at least one other person, usually my wife or one of my kids. Sometimes I grab friends if they know something about the content or setting of what I'm working on. They always think of or remember things that I wouldn't have. I am often surprised at how we tend to pay attention to different kinds of details.
When I wrote Love Puppies and Corner Kicks, a story about a girl soccer player moving from New Jersey to Scotland, I brainstormed with my wife by just asking her what she remembered about Scotland from our three years of teaching there. I hit up my daughter about her soccer playing days too. I picked up a lot of material that made it's way into the book. It was productive and fun too!
I'm always bouncing things of off them and reading aloud passages to get their reactions. My brainstorming crew don't seem to mind these sessions, maybe because when the book comes out, they know there's some of themselves in there. It's all good, as long as they don't ask for a cut of the royalties. :)