The largest military snowball fight on record was between Confederate troops during the Civil War. After a snow storm hit Virginia's Rappahannock Valley in January 1863, members of the First and Fourth Texas Infantry started a friendly snowball fight with members of the Fifth Texas. After the Fifth drove them back, the group decided to launch a snowball attack on the Third Arkansas. This fight devolved into a battle with members of soldiers from Arkansas, Texas, Georgia, and the Army of Northern Virginia. According to the history of the Fourth Texas Infantry, it is estimated more than 10,000 soldiers took part in the battle.
The blizzard that impacted the east coast in February 2010 led to a 2,000 person snowball fight in Washington, D.C.'s Dupont Circle. This was unique because of how it was organized: using Facebook and Twitter.
Collegiately, two of the largest snowball fights are recorded to have happened in 2009. In October, 120 tons of snow was shipped to Leuven, Belgium for around 5,700 students for engage in a massive snowball fight.
A few months later, an estimated 4,000 students at the University of Wisconsin-Madison engaged in an arranged snowball fight that was helped by the school cancelling classes due to more than a foot of snow falling the night before.
The mayor of Taebaek City, South Korea organized a massive snowball fight to coordinate with the city's Mt. Taebaek Snow Festival. Guinness lists 5,387 people participated in the snowball fight, which is about 10% of the city's population.
But Seattle might take the world record for hosting the largest snowball fight. Organizers trucked 81 tons of snow out of the Cascade Mountains to the Seattle Center near the Space Needle for Seattle Snow Day January 12, 2013. According to Guinness (and their 130 judges), more than 5,800 people took part in the event and some people requiring medical attention after the fight.
If a group wanted to try beating the record Seattle may take from Taebaek City, they'll need the people and representatives from Guinness to certify the event.