Samantha Grigg told the court that she drove Dustyn Frolka, 19, to ATMs before two others beat and robbed him. Frolka was found unconscious Feb. 15 on I-69 in Clinton County's Bath Township, near Lansing.
The 18-year-old Grigg was originally charged with first-degree murder and armed robbery along with two other teens from Saline.
Grigg's lawyer, Ron Zawacki, earlier said a plea deal was expected. She agreed to testify and faces less than 15 years in prison when sentenced May 27.
She is expected to serve between three-and-a-half years and seven years in prison as a part of the deal, Zawacki said.
"She was basically given gas money to drive these people up," Zawacki said. "I think she probably didn't think about what the ultimate consequences of this could be."
First-degree murder is punishable by life in prison with no chance of parole. In addition to Grigg, 18-year-old Tyrel Bredernitz and 16-year-old Brendan Heim face charges in the case. Bredernitz, who earlier waived his right to a preliminary examination, faces a June 18 trial.
Heim is undergoing competency proceedings.
Grigg said Monday in Clinton County Circuit Court that she and the others facing charges traveled to East Lansing with plans to beat up and/or rob Frolka, a sophomore from Owosso. She didn't say how they knew him. She said the SUV was going between 45 and 60 miles per hour when Frolka jumped out.
About $800 was taken from Frolka from alongside the road, Zawacki said. Frolka died from his injuries after being taken to a Lansing hospital.