The group arrived at the house which was home to a friend of Kerr in Abertridwr just after 17.30 BST, the court heard.
Kerr had gone to the house to play Fortnite and smoke cannabis with friends.
It’s alleged Mears went into the house armed with the knife, that witnesses said was about 10 inches long, slightly curved with holes in it, purple and silver in colour.
Mears said “alright boys,” a greeting which appeared to be directed at Kerr, the court heard.
Mears was described as “acting like a gangster”, repeating the phrase “What you want?” while brandishing the knife.
He was followed by Palfrey carrying a wooden stick, which one witness thought might have had a nail in it.
The court heard that around 40 minutes before the attack, Palfrey had turned his mobile phone off.
Palfrey warned the other people in the living room not to get involved. Elliot remained in the hallway acting as a look out.
Prosecutor Michael Jones KC said the fatal attack on the teenager took less than two minutes.
He said witness Connor Davies described Mears “stabbing and swinging out” at Kerr who tried to push him away, before seeing the blade of the large knife going into his coat.
Jones said Palfrey watched the attack while holding the wooden stick above his head “as if warning the others not to get involved”.
Others in the house believed he was in charge as he told them: “Stay away, let them have it out.”
The prosecutor added that it was as if Palfrey was the foreman stopping Kerr’s friends getting involved.
“Tristan Kerr died at the scene as a consequence of that attack and that fatal blow deliberately inflicted on him,” he said.
“Although that fatal blow was delivered by Georgie Mears, all four of them bear the joint responsibility for, and are therefore guilty of his murder.”
The jury was told Manny “undoubtedly” saw that two of her passengers were pre-armed and walked into the house immediately after Kerr was fatally stabbed and acted as the getaway driver.
After fleeing the scene in Manny’s car, the four were later arrested in Hereford.
The trial continues.