'You guys' or 'You lot' is often used to show one is addressing a whole group, as in:
'What are you guys up to now?'
If the 'guys' is omitted then one can't easily discern whether 2nd person singular or plural is implied. It can, of course be said in a different manner, but then succinctness goes to the dogs.
This problem, like so many others, does not exist in other European languages.