Hello! Fellow American here, somewhere in the Eastern time zone (sorry, prefer to be very vague about location online).
Because our country is 1) so spread out and 2) a federal system of 50 states, it is really, really hard to organize! But efforts are ramping up.
Women's Liberation Front is a radical feminist group that has been fighting gender ideology probably the longest of the US organizations:
www.womensliberationfront.org
US chapter of Women's Declaration International, headed by the tenacious Kara Dansky, is newer, affiliated with the global organization, and not specifically radfem, I think:
womensdeclarationusa.com
(looks like there is a contact for Texas but not yet for NC: womensdeclarationusa.com/get-involved/state-contacts-map/ )
(link to recent event outside the White House which they coordinated with Kellie-Jay Keen/Standing for Women:
Two very active grassroots protest groups I like are Terf Collective and RevFem Rebellion. They are both on Twitter but I'm not sure how else to get in touch with them.
Save Women's Sports does what it says on the tin, if that's a particular interest of yours:
savewomenssports.com
As far as online places to chat with other women, besides Ovarit there is also spinster.xyz. As far as I know Spinster does no censorship at all of women's speech.
Good luck! I used to post here more often when I lived in the UK, but since moving back to the States I've been more active offline. I still read here almost every day, though, so if we gather a good US contingent I'd be thrilled to post here more actively again.
I love the old-school message board format here; find the newer-style social media with followers and upvotes and so forth a bit annoying and hard to read. Plus Mumsnet vipers are the FUNNIEST and SHARPEST! I like the sense of hanging out with UK friends virtually. 
Just don't get them started on Weetabix...