The phrase "to know a guy" means having a connection or acquaintance who can help you with something specific or provide you with special favors, often because of their expertise or influence.
Sentences with «to know a guy»
You do know the guy who did that movie was tossed out his position at a college for lying, yes? (religion.blogs.cnn.com)
Then the two strangers admit they didn't know the guy who threw the party. (waytooindie.com)