"You" would be more formal or "correct". "You guys" or "You all" or "Y'all" would be more informal or colloquial, and usage would probably depend on region, though any of these would be easily understood anywhere in the US. (I'm an American English speaker who has lived in various parts of the US.)

When addressing multiple people, it is more common to say "you guys" in colloquial speech. "Y'all" is more Texas based, but I think most people in the US would understand that you are saying "you all", so either is fine. Saying "you" is usually only referring to one person.

You can say "You" for both singular and plural. "You guys" is more like slang.

'you' is probably the best way to refer to multiple people, as in "Are you going to the movies?" "You guys", "Youse guys" (in the New York City area) and "Y'all" (in the south of the USA) are also ways of referring to groups of people but they are colloquialisms which can be particular to a region.

The possibilities are endless!