It's just slightly tricky to translate ihr into English exactly. You are correct in that, most of the time, you would just translate it as "you" and lose the extra meaning that it's referring to multiple people. However, there are situations where you would possibly want to emphasise the 'multiple people' nature of the phrase in English - for example, you wish it to be understood that you are talking to the group as a whole, not just one person you happen to make eye contact with. In this situation, because English doesn't have the du/ihr distinction, you'd need to add in "all". This is not modifying the meaning or emphasis from the original German sentence, and so it is a perfectly good translation. Of course, the sentence should also be accepted without "all".