The angry teens arrived at the scene after a 16-year-old girl, who heard screams coming from her mother's bedroom on June 6 and thought she was in danger, called them instead of the police.
They entered the woman's Connecticut bedroom, and one of the teens used a baseball bat to beat up 25-year-old Roger Swanson, while others used their fists.
The victim was hospitalised with a black eye and several bruises.
"They knocked on the door and were let in," the New York Daily News quoted Torrington Police Lt. Bruce Whiteley as saying.
"Swanson and Arnold had not been fighting," he said.
The girl's mother, Melanie Arnold, 34, denies screaming and says that her daughter only heard a slap.
Only one of the teenagers was over 17, and they were arraigned in Bantam Superior Court in Torrington July 1.
Dilyen Langdeau, 19, was charged with third-degree assault and conspiracy to commit assault.
Police also charged the 16-year-old girl with assault and carrying a dangerous weapon.
Two 17-year-old boys were also charged, one with assault and the other with conspiracy to commit assault.