I don't have any tips but I'm also in this situation and would like to know. I asked a union organizer once and all he said was "that's a tough situation" and internally I was like "gee thanks I already knew that".
I think one option is to seize on a galvanizing moment like a round of redundancies, contract change or merger and use this to get people in to a private group where they can express their opinions with less fear of reprisals.
Of course you would still have to initially communicate via some company owned channel. For this I've found people are more willing to talk one on one via voice/webcam than text. Make sure any transcription feature is disabled.
Obviously a big problem here is that you are already on the backfoot since you are relying on the the company's actions to create the right conditions. You can try instead be proactive with these chats but the momentum is much weaker.
I've been in my office twice and I've been employed there over a year now, but I still joined a union. I'm in Germany though, so this might not be very helpful for you. Unions are quite strong here. And the union already existed before.
Use unofficial emails that are for personal social matters.
Also while Corporate may own Slack or Teams they wouldn't have ownership of a discord server where the employees just hang out to talk about off topic memes.
Simply put have a digital space that is outside the corporate infrastructure.
I think getting people on to a neutral platform is definitely the goal. In my experience though many remote workers just do their jobs and keep to themselves so enticing them to these spaces is difficult. The ones I've had best luck building solidarity with are those that have attended at least one in-person event and built rapport.
What tactics do you think would work to win the others over in this environment?
Maybe create a separate chat server for the workers to organize on?
What do you think would be a good way to get people to join?
A good question. Emails, mention its existence in video calls, etc. It's a risk of course to do it on company-controlled communication channels, which is all there really is in remote settings
Problem is remote workers are all over the place in as many jurisdictions. Unless you're part of a national union it's gonna make things difficult.
I have no idea really, but my idea would be to create an excuse to get people on another chat. Host a party, movie night, or something, and tell anyone who wants to go to join this other server. Even better might be a game night, and you can add them on Discord or something. Basically, your first goal probably shouldn't be the union. Your first goal should be getting them into a casual chat group that isn't owned by the company. Then you can create smaller groups for people who are vetted and start pushing for a union.