"The masculine personal plural adjective ending is -y/-i and the preceding consonant is softened: dobry → dobrzy, ładny → ładni, miły → mili, wielki → wielcy, drogi → drodzy; for more examples, see below."
(it goes through how various stem consonants are softened, including)
"w łatwy easy = łatwi"
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 780-782). McGraw-Hill Education. Kindle Edition.
In another section, it said how w softens to w', which represents wi
It depends. If the guys are young enough, actually "chłopaki" can work well.
Another word is "faceci" (sg. "facet")
Sometimes also "goście" (sg. "gość") is used. That's a colloquialism for "guys", but a normal word for "guests/visitors".
Note that all those are definitely masculine. In English you can sometimes use "guys" for a mixed group or even for a group of women. I think that you could then either use "ludzie" (people) or "dziewczyny" (girls).