They shouldn't necessarily be over at Truth Social. They should - eventually - be wherever they think effective resistance can best be mounted. More about that in a moment.
But they also should have a "home base" - a place where they can plan and organize. Where they can agree on where to focus their resistance efforts. Since the Internet is a thing, that place should probably be an online forum with a strong sense of identity and powerful tools for silencing and/or ejecting voices that aren't part of the team. It sounds ugly, but that's the way it has to be.
I'd have thought Democratic Underground (2.0?) would be a better place for such plotting, but kids these days...
... Anyway, I suspect that resisting Trump will largely have to take place in the real world, not online. Charlie Kirk's Turning Point USA movement goes to college campuses and engages students in conversation. You want to resist his influence, show up to those conversations. Join the local Parent-Teacher Association. Show up to school board meetings. Show up to city council meetings. Wherever ideas are being converted into actions, get involved. Whether with brickbats and molotovs, or heated rhetoric, or gentle remonstration, get out from behind the keyboard and show up.
But, you know, go ahead and hatch your plot on social media first. Why not?
ETA: Or just join your local antifa clique. I suspect the serious resisters probably already have their safe spaces for plotting. Spaces where everyone is a trusted operative, and they feel safe enough to plot the more serious acts of resistance.