Hate to disagree with you but on close inspection of the dialogue I have to conclude they didn't know each other. My guess is it is a pizza place close to the bars in their city. Obviously the camera man is heavily intoxicated based on his slurring, I am guessing everyone else was as well. It was likely late in the night after they all got yanked at separate places.
Pizza slapper and cameraman continued to insult at provoke after the slap, leading me to believe it was not exactly horsing around between friends. "You're ugly as fuck" is pretty harsh after the pizza slap. Then "fucking






s" and "keep looking at us like you're mother fuckers" to cap it all off. Dudes were out to humiliate.
My guess is that they are hostile idiots to begin with. My guess is they saw Marco's group coming in smiling, happy, and loud with good looking girls. These two guys, being alone with each other at the end of the night are bitter and jealous. So they decide to make an agressive move on one of the dudes to try and punk him and regain some manhood.
My guess is Marco took the handshake because he was drunk at the end of the night and just having a good time. Probably wasn't sure if he knew the guy or not so took the handshake anyway. Even if he was a stranger I have had plenty of strangers hug and embrace me when I've been out drinking. I've just gone with it because having a stranger put hands on me in a friendly but overly familiar way is not really reason to start an issue with both parties drinking. I feel most people probably agree with me.
Final bit of evidence is the girl's reply to cameraman's pick up attempts "I just want to make sure he's okay, our friend did this to him." If they knew each other they would not need to let them know she was friends with the puncher.