The term is related to but predates griefer, and was reserved for players disruptive only to their own team. A llama is different from a noob in that the player is not new to the game but in fact uses their familiarity with the game to purposefully harass their own team.
Probably one of the most well-known examples of llama behavior, crouching in front of the exit to your team's respawn in TFC used to prevent teammates from leaving the respawn room but kept the door open for enemies to shoot/spam grenades through the door, leaving your team defenseless.
snowballing loogie llama llamad
Often, a Llama will begin with a larger amount of people (7-10), but slowly drop off as the night progresses.
Sarah "Then don't. I'm having a llama tonight, come to that instead."
Jonathan "Okay. Who's coming?"
Sarah "So far? Me and you."