What resistance? It seems to me that there is not one big organised resistance that is taking members.
Find a local event and network but be careful about it. The US had that "no kings" thing early in the summer (or before?). That was just a bunch of people against tyranny. No real organisation to it. So you'd go to something like that and just talk to people.
It helps if you have an anonymous way to chat. Something like Signal or Matrix or one of the others. Even Telegram would be better than using something public or corporate-backed, like SMS for the former or WhatsApp for the latter. But be careful with Telegram, read up on it, it's not a solid recommendation but it's better than nothing. Fortunately there are alternatives. Even if the other person isn't tech savvy โ you probably are by being on Lemmy. So, show them the way.
Occasionally contact them through anonymous chat, just see how they're doing, and discuss future plans.
Also, aside from demonstrations, places where like minded people meet. For the authoritarians and conservatives, that's church, the local BBQ spot, and other, more obvious groups. I feel like they feel they can operate more openly since the US president is one of them. Used to be, these guys wore masks and were hidden, joining them wasn't about finding them, they'd find you, that kind of thing. Nowadays it's all out in the open. But I think liberals and progressives have meeting places, too. In response to church, for example, you should know that The Satanic Temple is not about devil worship, they're about resisting Christian imperialism. They're the ones fighting churches trying to get the Ten Commandments posted in every classroom. I'm religiously neutral, so they do not interest me, but they're certainly an option. For the anti-theist/"hard atheist," they're a good option.