Los Angeles police said the shooting happened in the 12400 block of Longacre Avenue at about 9 p.m.
Friends of Justin Lishey, who was well known in the Los Angeles hip-hop community as Kid Cali, and more recently Cali OSo, said he was shot and killed in the backyard during the party.
"We just heard gunshots, everybody instantly just ducked and everybody started running," Dorthea DeVilla recalled.
DeVilla said she and the 30-year-old rapper were close friends.
"Music is his life, that's all he talked about," she said. "I lost a really, really, really good person in my life and it's like... it really hasn't registered."
DeVilla said friends told her Lishey was involved in some sort of altercation at some point in the party.
Authorities said Lishey and two others were shot. The other two victims were listed in stable condition, according to officials.
Lishey was rushed to the hospital where he died.
DeVilla described Lishey as someone who got along with everyone, and said she couldn't fathom why anyone would shoot him.
"He's a great person, he has a good heart and he's going to be missed," DeVilla said. "He's going to be missed, not just by me, but by everybody."
Rapper Snoop Dogg paid tribute to Lishey on his Instagram, posting a photo of Lishey and writing, "@kidcali_oso rest well young g."
Investigators said the party was advertised on social media and those attending paid an entrance fee. More than 100 people were at the party, according to officials.
The gunman ran off from the party after the shooting, police said, but a description of the suspect was not released.