Theres really no way to answer this specifically. It all depends upon the dynamic. But in general here's how I handle "committee" assignments.....
It's a given with most designers that "design by committee" is no fun. However, input from a committee is much better than input from a single individual. As a designer I find that having as much input from every source which will be reviewing my work is most helpful. I would hesitate to lock out all voices in favor of one. I often sit in on meetings to simply listen to everyone even if there's no direct design related input. I've sat through three-way phone conferences and listen to people argue about things just to gain insight into company dynamics. Basically, I try and soak up as much input as possible. By eliminating all those voices you eliminate some possible insight which may be helpful to the design processes.
I would suggest being a part of the committee meetings in the initial stages so you can hear everyone's ideas or thoughts. If you stick to one voice, it's most likely you'll miss something. And what you miss may the one thing which sparks a winning idea.
Take all that committee input and work up preliminaries. Present preliminaries to the committee and get feedback.
Then, from that point, eliminate the committee in favor of one voice. You'll have all the input which will be valuable by this point. Any committee input from this point forward will simply be disagreements within the committee and it is their issue to sort those out. Have them designate one individual for final approval. Then deal with that person from that point forward.
Essentially... A committee is excellent for initial stages. A committee is unworkable for final approval.