"Did you all work yesterday?"
Translation:Avete lavorato ieri?
Shouldn't this past participle form agree in number and gender with the subject (ie, shouldn't it be lavorati, lavorate)?
No, that's only when a verb takes essere rather than avere.
The only time you change the endings of the past participle with verbs taking avere is when you have a preceding direct object, when the ending agrees with that object, as in, L'ho trovata, I have found her, or, Li ho trovati, I have found them.
Eh... And where is the word "all" in the correct translation "Avete lavorato ieri?"?
It's an Americanism. They often say 'you all' when they mean 'you' plural. It doesn't have the emphasis 'did all of you (no slackers) work yesterday' that it would in British English.