Vet /screen everyone. Cross check personal references, have an onboarding period before introducing new members to anything sensitive, do background checks if you’re concerned about it. There are already experienced community leaders and activists in your network; ask them how they establish trust. Teach people to think more critically about how and why they trust information and people. Make vetting part of recruitment and onboarding processes, and make recruitment work a self sustaining perpetual process with a team of people to support it. After you establish trust, use it. Ask around for feedback about new members.

Accept that you aren't going to always outsmart infiltrators and guard your information silos accordingly. Have processes for auditing threats, removing members. Avoid creating large group chats or exposing names/ contacts without consent. Keep working groups small, and don’t include people who aren’t actively participating. Use encryption, use pseudonyms. Use the basic tools available (Signal, GrapheneOS, Tails, Tor, etc). Don’t retain things like chat history or files you don't need. Give people basic training on social engineering techniques, cyber security (or honestly just how to use a computer 101), operational security, and how to recruit.

Ultimately, retain a good legal defense. You probably aren't doing anything illegal anyway, but that doesn’t mean you shouldn’t have a good lawyer and a plan… legal ground is shifting, and you will never have more resources than a motivated agency task force. Get a dash camera, and non-evil doorbell camera. Have a group of friends who will check in on you regularly. If you are discussing illegal shit, don’t do it around people or devices you don’t completely trust. Retain plausible deniability.

Know what is and isn't legal protest and keep a sharp eye on anybody offering a hookup to explosives / rocket launchers, etc.

( probably a three letter agency plant )

Take playbooks from large criminal organizations. Protesting may not be criminal, but it is increasingly being treated as such.

Organized crime deals with this stuff constantly. Meet in person. Vet everyone. Being willing to keep lips tight and risk imprisonment.

Posting all over BlueSky is just snitching on yourself. It doesn’t matter if protesting is a cornerstone of our democracy when those in charge don’t care. As the water heats up, protestors will need to reassess how they view their own actions.

The primary downside is that operating like this sucks. It’s inconvenient at best. It requires modifying your own behavior.

Especially since it isn't spying.

I think if all protesters really just protested (and nobody tried smashing ICE cars tail lights, physically blocking their vehicles, and damaging officers' hearing), that would go a long way helping prevent this. So the measure protesters can take is to discipline their ranks.