In the Devoto-Oli online dictionary, under "collaborare", I find this example:
I cittadini collaborino a mantenere pulita la città.
I don't understand why that's congiuntivo. I read it as a straight-forward affirmative statement. Or is it a (formal) imperative, something like: "Citizens! Keep your city clean!" But shouldn't it then begin with a "Che"?
The sentence “I cittadini collaborino a mantenere pulita la città” is not a statement of fact like “La Repubblica riconosce e garantisce i diritti inviolabili dell'uomo” (article 2 of the Italian Constitution). It is rather an exhortation to the citizens to keep the city clean.
Not really an imperative (Italian properly has imperative only for the second person, singular or plural), but rather “congiuntivo esortativo”. As the article in the Enciclopedia Treccani says, this form of subjunctive also supplies for the missing forms of the imperative. The context helps in distinguishing between an exhortation and an order.
So the meaning is quite likely “Please, citizens, cooperate in keeping the city clean”.
The sentence “I cittadini collaborano a tenere pulita la città” might be a statement of fact (somebody notices that the people do work for keeping the city clean) or, like for the Constitution, a statement of a duty. As usual, context helps.
