California Highway Patrol investigators said Sylvia Garay, 18, was drunk last July when her SUV hit a concrete barrier and flipped over on Highway 99.
A Greyhound bus hit the SUV and went down an embankment. Garay, her two passengers, and three people on the bus were killed.
A CHP report placed sole blame for the crash on Garay. The report said the bus driver had no way to avoid the SUV, which was left without lights when it overturned.
Michelle Kay Cole, 22, is being charged with providing alcohol to a teenager resulting in death.
According to the report, Cole - a friend of Garay - went into a liquor store with Garay and her two passengers and bought two bottles of flavored vodka. It said Cole used her valid California identification card to buy the alcohol.
Cole knew her friends were underage, investigators said. Garay and her passengers later drank the alcohol at a friend's house, they said.
Cole is due in court on Aug. 23 for arraignment. She could face six to 12 months in jail and a $1,000 fine if convicted.
The Associated Press contributed to this report.