Left Unity said Labour should be "thrilled" so many people want to join rather than focusing on "people who it thinks are too left wing".
It comes after The Times reported "infiltration" of the contest to replace Ed Miliband by the "hard left".
Labour says it has a "robust system" to prevent "malicious applications".
Ballot papers have not yet been sent out in the leadership contest, which pits Mr Corbyn against Yvette Cooper, Liz Kendall and Andy Burnham.
Labour says the verification process is ongoing, and has refuted claims the leadership contest is being infiltrated by those who do not support the party.
Acting leader Harriet Harman emailed MPs this week with a list of new members from their constituencies, asking them to check for any suspicious names.
As well as regular members, people can sign up as registered supporters for Â£3 and take part in the vote. They are asked to confirm they "support the aims and values of the Labour Party".
The Times reported 18 members of Left Unity, a left-wing party founded by film director Ken Loach which fielded 10 candidates at the general election, had registered.
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Left Unity principal speaker Salman Shaheen said: "Left Unity is a party with many disaffected former Labour members. It's no wonder that some people genuinely wanted to have a say in the Labour leadership contest.
"They fully agree with the aims and values of the Labour Party - it's the Labour leadership who abandoned those values."
Mr Shaheen said it was "ludicrous" to refer to its supports as "entryists", a term used when members of a political group infiltrate another in order to subvert its policies.
"The surge for Corbyn is a genuine popular movement that the Blairites can't stop," Mr Shaheen added.
"It's not about us - they found just 18 Left Unity members out of the tens of thousands who have signed up to support Jeremy Corbyn's anti-austerity, true Labour politics."
Labour said: "All applications to join the Labour Party as a member, affiliate or supporter are verified and those who are identified by our verification team as being candidates, members or supporters of another political party will be denied a vote."
The apparent success of Mr Corbyn's campaign has prompted a row within the party, with other candidates saying a Labour Party led by him could not win an election.
But Conservative London mayor Boris Johnson said his party should "pay attention" to some of the things said by the Islington North MP, suggesting some of his analysis was "not wholly wrong".
"Yes, there is a problem of inequality. Yes, there is a problem of low pay," he said during an event in central London.
He added: "I happen to think his solutions are completely wrong and he would take the Labour Party in radically the wrong direction.
"But it would be very complacent and wrong to ignore the truth of some of the observations he is making about ways in which society could be better. We should be humble about that."
A left-wing political party has accused Labour of a "witch hunt" against people signing up to back Jeremy Corbyn in its leadership contest.