"To join the protest" means to participate in a public demonstration or event, expressing your support for a cause or voicing your disagreement with something. Full definition
Some school districts have threatened to suspend students who join protests for tougher gun laws. (insidehighered.com)
Educators, school workers and parents joined the protests to push for greater school funding. (npr.org)
He only began taking professional risks by working with Greenpeace and by joining the protest against Keystone when he came to the intellectual conclusion that climate change is a moral issue. (esquire.com)