Woodstock police have been investigating this incident for several months. They say the girl's father went to police after finding out about it on social media.
James Cornelius and Corion Simpson, both 18, have each been charged with criminal sexual assault four months after allegedly having sex with a 15-year-old girl. They are currently being held at the McHenry County Adult Correctional Facility.
Also charged are two juveniles, one with criminal sexual assault and the other with possession of child pornography.
Woodstock police say the alleged incident happened in December at a house party where there was no adult supervision and underage drinking. One of the suspects recorded the alleged incident and posted it on Facebook.
"The victim's father became aware of a social media posting of the event of a sexual assault and abuse that took place with his underage daughter," said Chief Robert Lowen, Woodstock Police Department.
Police say the girl's father came to them after finding out about it in February.
The four suspects are all students at Woodstock High School and were taken into custody on Tuesday.
Police say the 15-year old girl is from a neighboring town, but knows the suspects. Relatives of James Cornelius say the girl often travels to Woodstock.
The two juveniles were released to their parents. The two adults are due back in court next month.