A zerg guild is a term used in MMORPGs to describe a group of players with little to no skill, employing hundreds of accounts to get anything done. If an encounter requires 5 people, a zerg guild brings 20.
Offspring of the term 'zerg rush' which was coined from Starcraft, after a race in the game that employs such tactics regularly.
1: "Hye u got teh strat for uber_mob lol??///"
2: "Why? It would take 80 of you to kill it with your zerg guild."