I can suggest something like the BLM raised fist, it is used mostly around BLM but I think it’s a good symbol for activism as a whole?
Something like this: https://stock.adobe.com/nz/images/black-lives-matter-fist-hand-graphic-icon-symbol-black-power-illustration-perfect-for-a-protest-sign-card-or-shirt-design/366822987
Oh I like the megaphone one as well!
I was thinking about multiple people as well, but in the search and on mobile the icons are kinda small, idk if it will be visible. Maybe we can use it as a wallpaper(or however the banner behind the icon is called).