"You see us and them" is "Widzisz nas i ich" and "You give it to us and them" is "Dajesz to nam i im", while "Who wants a cake? Us and them!" is "Kto chce ciastko? My i oni!". Apparently the last example fits here, I cannot find another explanation.
Except that English tends to use the accusative forms of pronouns when there's no verb. So 'Who's coming?' - 'Us and them', never 'we and they', unless there's actually a verb following. As a free standing sentence, 'we and they' sounds wrong.