Have you researched? Without outing myself, there's local websites for my city that I use to find how to participate.
For more broad national one: https://www.teslatakedown.com/
Facebook unfortunately is still where a lot of folks go. Nextdoor, is another one. Heck, even craigslist has posts about upcoming protests.
Places with a local bulletin board, like coffee shops also have events.
And honestly, in this current climate, I'm suspicious of any polished app/website for protesting that could give away my identity as a attendee. Police have a lot of experience with keeping tabs on these things.