How would you say "herding cats" in Italian? I would like to know if there exist an equivalent proverb in Italian.
I’m afraid there isn’t a similar idiomatic expression in standard Italian.
I would use
È un’impresa disperata.
Which indicates that a task is strenuous yet pointless – it’s set for failure.
(However this is a more generic expression, so it wouldn’t be good if you’re specifically referring to trying to manage an uncooperative group).
The answer from Andrea is correct; I add that, less formally, the often used word is "casino" to define a situation difficult to sort out, like: "Capire mio fratello è un casino" ("trying to understand my brother is like herding cats").
This word "casino" (see Treccani vocabulary) can be used to indicate something disordered and uncontrollable, i.e. caos, like in "il traffico di Milano è un casino" or something difficult to do (because it is uncontrollable).