The group, which really isn't too secret anymore, is called Pantsuit Nation. It started so Clinton voters could talk about politics and support her on Facebook without judgment.
The group's creator, Libby Chamberlain, said in a statement to BuzzFeed News that she started the group after a conversation with a friend about Clinton's historic candidacy.
"We talked about how beautifully and stoically Hillary embodies women's fight for equality, and how the pantsuit is an emblem of that struggle," she said. "It's a symbol that might be lost on younger women, and so I wanted to do something to re-appropriate that symbol and everything that it means to me as a feminist and Clinton supporter."
Chamberlain said she decided on a "whim" to wear a pantsuit on election day, and invite her friends to do the same through a Facebook group. It has since exploded through her social circle and beyond.
She said she was shocked the group was so popular.
"I think the real feat of this group has been its ability to honor each member's personal story and celebrate all the reasons why this election is the most important of our lifetimes without resorting to the usual vitriol that seems to plague many other spaces these days," she said.
Many women are sharing personal stories about how it feels to vote for a woman president.
"I'm voting on Tuesday for all the strong women I've known in my life. My maternal grandmother would be so proud to see a woman president!" one woman wrote.
Others said they were swayed to vote for Clinton after being disgusted by Donald Trump's campaign.
"My mother has voted Republican in most elections, but stands with Hillary tomorrow," one woman said.
Group members are also sharing photos of themselves rocking a pantsuit just like HRC.
Chamberlain said she and other moderators are keeping the group positive by only approving personal and positive posts.
The group is also encouraging its members to donate to the Clinton campaign, and Chamberlain said they have raised $140,000 so far. She said the group hopes to continue even after Tuesday.
"For many, this group has become a safe space to share their stories and support one another; we’re hoping to continue this safe space post-election," she said.