The JC heard the JVL chair telling activists at the event last week that Mr Greenstein and Mr Wadsworth were victims of "appalling injustice".
Mr Greenstein was expelled by Labour in February over his use of antisemitic slurs, including the term “Zio”. Mr Wadsworth was stripped of his membership in April after he heckled Jewish Labour MP Ruth Smeeth at the launch of Labour’s antisemitism inquiry in 2016.
Other speakers at the meeting suggested a "British-Israel lobby" was behind repeated "smears".
It has been suggested that Mr McDonnell - Mr Corbyn's closest ally - was keen to stop Labour's antisemitism crisis escalating ahead of a possible snap general election.
But his decision to appear alongside the JVL chair suggests Mr McDonnell is now ready to go on the offensive and back the group over the more established communal bodies, such as the Jewish Labour Movement, the Jewish Leadership Council or the Board of Deputies - all of which have condemned Mr Corbyn over antisemitism in the party.
In an interview last week, Mr McDonnell spoke of the need to listen to "broader" views within the Jewish community.
A promotional message on social media by Barnet Momentum advertising the event with Mr McDonnell and Ms Manson says: "Come and join the future Chancellor of the Exchequer, unrepentant socialist and champion and defender of Barnet services John McDonnell in a candid one to one conversation with future Prospective Parliamentary Candidate, champion and defender of the oppressed, and opponent of all injustices, the determined and unapologetic Jenny Manson."
Ms Manson has previously told the JC she was "thinking" of stand as a Labour candidate but had not decided where.
