"You are all busy."
This is how I answered and it gave me another translation for both to be correct. I came to the comments to see if there was a reason, but nothing yet.
Yes, it is accepted (Jan 29, 2019) but I suspect it is customary to include it. According to dictionary 很 (hen) is an intensifier: quite, very, awfully.
You all is the same as All of you which translates to 你们都 and not just 你们 which means you plural