guy (pronounced gal) is used informally in English with the meaning of “dude”, as slang for so-and-so. For example:
"He is the guy we met some days ago." (He's the guy we met a few days ago.)
"He is a nice guy". (He's a nice guy.)
guys (pronounced gas) means “guys” or “personal”. It is normally used as a vocative to greet a group of people. The phrase "Hello guys!" can be translated as “Hello guys!”. The greeting is intended for all persons present.
As you know, in the English language the pronoun you means you or you, and may cause some confusion in the distinction between singular and plural usage. In some regions of English-speaking countries, the term guys has been used accompanied by the pronoun you to indicate the use of the plural. This new plural form is only used in slang. For example: "Come on, you guys!" (Come on guys!). The phrase is intended for the group and not just one person. The word guys present in the sentence has the function of transforming the pronoun you in plural.