Brian Altman QC, prosecuting, said Shakilus had been seeing the girl for six weeks.
He told the jury: "Shakilus had described her to his mother as 'beautiful' and he said to her that he was in love with her, he was even saying to his mother that he wanted to marry her, that she was his future wife."
But Joseph was still in love with McLean, who had dumped her, and "would have done anything to get him back," said Mr Altman.
"He [Danny McLean] had come to learn of her deception and by way of recompense she agreed to set up the hapless Shakilus Townsend in a honey trap with a lethal and tragic twist," he said.
On 3 July 2008, Joseph lured Shakilus to a cul-de-sac so he could be ambushed by the gang, Mr Altman said.
As the pair arrived in the area, the attackers were waiting for him and chased him as she kept in touch with McLean by mobile phone.
Shakilus was kicked and punched and struck with a baseball bat.
He died the next day as a result of a wound to his chest, which had "raked" across his liver causing a massive loss of blood, the court heard.
The defendants belonged to a gang in south London
Joseph later set about trying to "rub out" any trace of her relationship with Shakilus, deleting his online Bebo account and telling friends to erase his number from their phones.
In court she admitted agreeing to lead him into the ambush so he could get beaten up but said she did not realise he would be seriously hurt.
After the convictions, Shakilus's mother Nicola Dyer, 34, said: "He was popular and always had girlfriends, but the girl on trial was different.
"He really cared about her and I can't understand how she could have callously set him up and lured him to his death."
During the trial, McLean, from Thornton Heath, admitted under cross-examination that he routinely carried a knife, claiming he needed it to protect himself and his family.
Shakilus had himself been in trouble in the past for weapon possession and robbery and had been to see his youth offending team on the day of his death - but his mother said he was turning his life around.
The defendants will be sentenced at a later date.
CCTV shows the girl and her boyfriend walking away after the murder
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