After they received a message from the host saying they were welcome, the group set off.
Within minutes of their arrival, however, it was clear not everyone at the gathering felt the same.
Cooper said people rushed into the lounge of the Melbourne St flat where he saw a man holding Henderson by the throat.
When he tried to intervene, he told the jury, someone hit him in the face then continued "wind-milling overhand punches".
Cooper remembered tackling his assailant — who he believed to be Sem-Cheyne — to the ground before being kicked in the head, pulled up and punched twice more in the face by another man, then marched out of the house.
As he walked down the driveway, he heard glass smashing and footsteps behind him.
Just as Cooper reached the road he felt something impact the right side of his head.
"Then what's happened?" Crown prosecutor Mike Mika asked.
"I don't know. I was unconscious," he said.
Henderson said he saw his friend fall between two cars before he was chased from the scene.
Neither saw who inflicted the blow.
But the Crown said a group of women who were arriving at the party would give evidence of seeing Sem-Cheyne kicking and stomping on the victim.
Their testimony, defence counsel Anne Stevens QC said, would be pivotal.
The women, she told the jury, were there to see a friend and had thrust the blame on to Sem-Cheyne to divert the blame.
"They're lying to protect their friend," she said.
Cooper told the court he regained consciousness and was ushered over to a fence where he sat with Sem-Cheyne.
They apologised to each other about the scuffle inside the house, he said.
When Cooper reunited with Henderson, who had also been assaulted, they waited for emergency services to arrive.
Once in hospital, Cooper said he became violently ill and underwent a CT scan, which revealed he had sustained a complex skull fracture.
A fragment of bone had pierced the lining of the brain, the court heard, and surgeons operated.
In the dock, Cooper pointed out the large scar running down the right side of his head.
He said he was now "mostly well" but had been left with a "slight stutter now and again".
The trial, before Judge John Macdonald, will hear from a dozen witnesses who attended the party and is scheduled to last four days.
