A 'hard worker' is just that. A person who works hard. 'Just' here means something like 'simply', or 'only'. It means that you should take one/some if you like. You don't need to do anything else. You don't need to ask for permission. 'Just' doesn't' really add much to the meaning of the sentence, but it is common to start an offer like this - perhaps because if you miss hearing the first word, it doesn't matter?