On the day of the attack Betts-Ramsey, Harris, Osei, Hinds and the 17-year-old boy waited for the victim at a bus stop near to Harrow Leisure Centre.
As the victim got off the bus at 18:51 GMT she saw the group of teenagers running along Masons Avenue in the direction of a car park and followed, the court heard.
“When she arrived, she was immediately attacked,” Ms Heer said. “The victim recalls that she had no time to say anything or do anything to defend herself.
“The assault was filmed by others from the snapchat group on their mobile phones. These videos were then posted on social media.”
Footage played to the court showed the victim being floored by several punches thrown by the teenagers who had surrounded her.
Ms Heer said once the victim was on the ground, Betts-Ramsey took out a knife and began stabbing her, while the others continued to kick, punch, stamp and drag her along the floor.
