The actress, 45, told News Corp it was "very painful" not being asked to join.

Some of Time’s Up most prominent leaders are responding to Thandie Newton after the Westworld star accused the movement of not wanting her participation.

The actress, 45, who has previously spoken out about being sexually assaulted by a casting director, recently told The Telegraph it was “very painful” not allegedly not being asked to join the movement.

“I have been an activist for 20 years. It’s been a long time coming,” said Newton. “I certainly agitated for it to happen sooner, but no one was interested and people thought I was a bummer. Even my publicist told me to stop talking about it because it was bad for my reputation.

“When Time’s Up was put together, to not be invited to be a part of it, was very, very painful,” she continued. “It’s hard for me, as someone who has been talking about it for a long time … it’s lonely.”

On Monday, Jessica Chastain, one of the movement’s most high-profile leaders, responded online to Asia Argento, who tweeted an article about Newton’s comments.

“I have felt so bad that some feel excluded,” she wrote in reply to Argento. “I was never asked to join anything, I just came forward to see how I could be of service. No one is unwelcome. The more women that come together, standing shoulder to shoulder, the stronger we are.”

She added, “You, and so many others, were so brave to speak up & start this movement. It is my goal to make sure it will never be in vain. I will always support you and what you did.”

Amber Tamblyn, a Time’s Up supporter who has spoken out against sexual harassment and abuse in Hollywood, replied to Chastain, writing, “100% what Jessica said. None of us needed an invitation to do the work. We just did the work. Period. This is such a disheartening dialogue to be having with fellow women. Such a shame.”

Chastain also responded directly to Argento, saying, “I invited you to the first meeting at my house Nov 27th but unfortunately you were out of town. It wasn’t called #TimesUp cuz we didn’t have a name. It was just a group of actresses coming together to see how we could support the brave women coming forward & create lasting change.”

Argento replied, “You’ve been so kind and supportive not only to me, but to a lot of survivors. But when you invited me to your house you said it was just a lunch with actresses, not that you guys were starting a movement.”

Argento has accused disgraced mogul Harvey Weinstein of sexual assault.

RELATED VIDEO: Women of Hollywood Attend Golden Globes Together in Support of the Time’s Up Movement

The Time’s Up movement, which launched in January to help fight systemic sexual harassment in Hollywood and beyond, has raised over $13 million for a legal defense fund — mostly thanks to generous donations by some of the biggest names in the industry.

“Anyone and everyone is welcome to join the movement. And it’s not just Hollywood, it’s for every woman in every Industry,” a source close to Time’s Up tells PEOPLE.

Over 300 prominent actresses and female agents, writers, directors, producers and entertainment executives signed an impassioned open letter that officially announced the movement on New Year’s Day.

(L-R) Jessica Chastain, Thandie Newton and Amber Tamblyn.
| Credit: Kevork Djansezian/NBC/NBCU Photo Bank via Getty; John Lamparski/WireImage; Andrew Toth/Getty Images

The open letter noted that the group is leaderless and rather works collectively to help work toward a solution. Time’s Up initiatives include the legal defense fund to help less privileged women, a mission to reach gender parity at studios and talent agencies and legislation to penalize companies that tolerate harassment.

Time’s Up began meeting in early October after a wave of allegations was spurred by a slew of stories alleging sexual misconduct by Weinstein.

A spokesperson for Weinstein previously told PEOPLE in a statement that “any allegations of non-consensual sex are unequivocally denied by Mr. Weinstein. Mr. Weinstein has further confirmed that there were never any acts of retaliation against any women for refusing his advances.”