Most people taking part in a ground breaking ceremony

According to Official Guinness Records,

The most participants in a ground breaking ceremony is 6,256 achieved by the San Jose Earthquakes on the grounds of their new stadium in San Jose, California, USA, on 21 October 2012.
A member of Major League Soccer (USA), the Earthquakes provided 6,000 shovels for fans of the team, as well as allowing additional fans to bring their own, to break ground on the pitch of its new football (soccer)-specific stadium.

For a complete list of 2012 records, please visit 2012 Guinness Records in United States.

Other United States records that might interest you:

• Highest driver score on Blur
• Fastest standing mile – car
• Oldest person to complete the IRONMAN®World Championship
• Most costume changes in eight hours
• Largest air drum ensemble
• Longest stoppie (endo;front-wheel wheelie) on motorcycle
• Largest Frito pie