I had encountered a greifer player who joined as Louis. He started killing Zoey and voted to restart the campaign, I tried to kick him but failed. He tried to kick me but the other players voted No. He was then voted to be kicked after I had killed Louis (so the player couldn't do anything). I was playing Blood Harvest, got to chapter 4 and on Expert diffcuilty. I currently blocked the player. If you see this user, be careful.
Some players that join could be a greifer. It's hard to tell if the player is a greifer or not unless you know about the player's behavior. Wikipedia has more information about greifers[en.wikipedia.org].
So far, this is what I think greifers will attempt to do. *Annoy other players through spamming such as vocalize spamming, mic spamming etc. *Waste items such Pipe Bombs, Pills, Molotovs, etc. *Attempting to kick an innocent player for no reason. *Intentionally shooting other Survivors until that Survivor is dead. *Killing oneself.
Friendly Fire in Left 4 Dead 2 is a problem. Many greifers will often team kill other survivors often to prevent that player from playing, this is common on Expert difficulty as you can easily kill Survivors. Greifers are one of the main issues I have in online games including L4D2.