What is my brainstorming process? Do I solicit opinions? Do I drag trusted coconspirators through the twists and webs of my weird?
I will bounce the idea of which project(s) to pursue off a friend or two, but when it comes to the stories themselves, that's not a group-think thing. It's not because I think I'm some sort of fantasy genius; it's more that my author-voice is rooted in how I conceive and farm the story. I need to be able to roll around and bury myself in my imaginary dirt without permission or supervision, or feeling like I'm intruding on someone else's turf. Even though I'm a skeletal plotter, I return often to the landfill of my imagination for the details of the story.
For me, story ideas usually start with a protagonist, two or three supporting characters, a couple of climactic moments, and an emotional challenge. From there it's figuring out magic systems, the presence of creatures, and the environment. Then comes the tricky bit, the plot.
Now, I do love to participate in brainstorming other people's ideas or just brainstorming with friends for shits and giggles. Makes me a fine hypocrite, I suppose, but what's not to love about a lengthy game of What If? It's a great way to learn more about my friends. And, who knows, I might even track a little dirt home.