People sometimes see working groups as the answer to the formal requirements of a committee but in reality there are limited circumstances where they are appropriate. They cannot make decisions but can include members of the public or non councillors. I've often found that unless they have clear terms of reference, have an agenda and clear direction, there's a tendency to go "off piste", forget the reason they were formed and fail to achieve anything. We generally agree who the participants are in a meeting (committee or full council, depends upon the reason for setting up the working group in the first place) so membership is open and transparent.