In July 2015, McGrath showed "unusual interest" in the penis of one of the complainants.
He allegedly instructed the boy how to clean his penis in the shower and McGrath told him he could show him how to clean it.
Soon after that conversation, the 14-year-old injured his groin during a game and McGrath escorted him to the changing rooms "under the guise of giving him some first aid for the injury", Webby told the court.
The coach allegedly told the boy to pull down his pants then started to rub a soap-like substance on his groin area and told him his pubic hair would grow as he got older.
Webby said the conversation then turned to masturbation and McGrath asked if he did it in bed.
The boy then "effectively" masturbated in front of McGrath while following his instructions, the prosecutor told the jury.
The next month, after the boy had missed out on getting into a representative team, McGrath allegedly told him it was because the other players were more mature.
Webby said McGrath told the boy if he masturbated in front of him "that would help him become more mature mentally and physically".
"You'll hear how this young boy trusted and believed what this defendant was saying to him," he said.
McGrath then allegedly sexually violated the teenager before putting his arm around him to stop him from leaving.
"He hugged him and told him don't tell anyone, this can be our secret."
McGrath gave the teenager $10.
"Unfortunately these weren't isolated incidents," the prosecutor said.
As a result of the police investigation, another boy was spoken to who also said he'd been sexually abused by the coach.
Webby said in 2013 during a weigh-in with his own grandson and another child, he instructed both boys to take off their shorts and underwear before stepping on the scales.
McGrath's grandson went first then left the changing rooms, leaving the coach and the 11-year-old alone.
The coach allegeldy asked if the boy masturbated and told him it was a good way to relieve stress. He told the boy to do it in front of him.
"Not knowing how to handle the situation you'll hear how he did what he was told."
Webby told the jurors they would hear evidence from the boys, their families and the police officer who investigated the case.
McGrath's defence lawyer, Siobhan Buckley, told the jurors it was the rules she could only give them a short address until the Crown had closed its case.
But said she wanted to make it clear McGrath didn't accept any of the allegations.
"In some fifty years with the club, he never inappropriately touched any young boy."
Buckley said while she didn't need to outline the defence strategy, McGrath would give evidence.
"The reason I'm telling you that is because the critical thing for you as jurors ... is to keep an open mind."
This afternoon, the jurors will be taken to see the changing rooms.
Judge Brooke Gibson told the jury this was an aide to assist them in understanding the trial "as it unfolds".
He also told them to keep an open mind until the end of the trial.
The trial is set down for two weeks and the complainants will start giving evidence tomorrow.
