Yeah Signal should be fine. Don’t use a picture, don’t use your name, activate disappearing messages, change your user name whenever you’re sharing your contact details. Never share your phone number, only the handle.
Activism is trust based, as others said it’s a lot word of mouth how you learn about things going on. The unfortunate reality is that depending on what kind of activism you’re doing there might be people infiltrating.
You have to be more careful if you’re e.g. a climate or pro-Palestine activist. This is my experience from Palestine activism, the Zionists actually send people speaking perfect Arabic into protestor groups in and outside of Palestine. They get the names & info on everyone, what their role is in the group, things they share, they use that to disrupt the activism, blackmail and jail people. So you might consider not sharing social media accounts and your real name either, the less the better. But that really depends on the kind of activism you’re doing and the kind of people you’re surrounded with. I would assume that climate activism might be equally dangerous. Although mind you my experience is also a dictatorship like scenario, or whatever you wanna call military occupation.
The infiltrator issue also sucks because it destroys trust, there have been cases where undercover cops lived a double life for years just to keep some group infiltrated but those are rare. And that’s why a lot of things are word of mouth, you get "in" over doing more work and building friendships with people.
Wow, fascists, whether corporate or colonial or otherwise nationalistic really know no boundaries.
In our case we had a guy in the group who claimed to be from a specific place I happen to know very well since I have friends from there and the dialect of the guy just didn’t match his story. This is something very specific to Arabic dialects though where every region, every city has their own dialect. It was very awkward to tell this to the others, because it just ruins the trust in the group.
That's why it’s often better to give menial tasks like putting up posters or painting banners to suspected infiltrators rather than put the accusation out there and keep them out of the loop otherwise.
In general I’d recommend having several layers of communication and only let those in which seem trustworthy. If infiltrators start having sexual relationships with group members and become leaders of the group like in the article, well… No idea what to do in such a case, this is just crazy.