It happened at the Anatole Apartments on the South Loop near the North Freeway Tuesday afternoon.
Matthew Davis has been charged with murder and manufacturing or delivery of a controlled substance. Joseph Wilson was charged with tampering with physical evidence.
Police say Davis brought the gun, and ultimately fired it, hitting the girl above the shoulders.
Wilson was with them in the apartment, and after the shooting, took time to hide drugs instead of calling 911, police said.
"It's pretty disconcerting. It doesn't just happen, you know," said neighbor Aaron Christian.
Police describe a drug-fueled party that quickly disbanded after the shooting.
They say Davis ran away from the scene, but witnesses helped locate him.
"It was just creepy, so we actually went and stayed somewhere else because I was just like sick to my stomach," said neighbor Annmari Cooper.
The investigation is ongoing.