guys is plural... "Look at those guys over there"
guy's is singular possesive.... "I stole some guy's watch"
guys' is plural posessive.... "Guys' Poker Night"
unless the host is names "Guy"
Then
"Guy's Poker Night" is correct because it refers to the singular not plural/
UNLESS....
The poker night is ONLY for guys named GUY hosted, of course, by a guy named Guy...
Guy's Guy Poker Night
But if they wear costumes that make them look like guys named Guy (male only, please)
Guy's disguised Guys guys' Poker Night
Unless they are from `Jersey///
Poker Party for Deez Guise what's dressed up like sumguy named Guy