"It's like bagging fog."

Dunno, FB, I think "herding cats" is a much better visual, especially if you're talking about trying to get a bunch of people to pull together. Says something about the people involved as well as about the futility of the task.

"Bagging fog" just implies the futility, although it does imply that very well.

"It's like giving a cat a pill" would be another good description of a futile task - one that would probably result in injury to all non-cats present.