The problem is any meaningful way to organize online will inevitably draw agent provocateurs and/or be infiltrated by the feds/police (known fascist supporters).

So that then requires you to recruit in person in places where you're most likely to find like-minded individuals (colleges, punk rock shows, alt lifestyle bars/clubs, lgbt friendly areas, low to moderate income urban communities, anarchist clubs, etc.).

When you do this you also need to emphasize and enforce decentralization of information within your growing group (even leadership) so if one person gets flipped/captured they cannot rat on enough people to bring down the whole organization. This is critical going forward, fuck this up and you're screwed.

Another thing to do is to NEVER ever let people use their real names or openly associate with your group, no patches, no pins, no tats, no stickers, etc. With the surveillance state this info can and will be used to track you and take you down, oh and that also means no cellphones.

Speaking of cellphones, most popular chat apps are compromised, so you'd need to figure out a highly encrypted chat app to use. Whenever your group is "out and about" strict ban on cellphones, use encrypted radios and don't give them to everyone either.

This is a very brief TL;DR about how to make and maintain a secret group of any kind. What you choose to do with that group is up to you (notice how I never mention what you should do with it).

the philosophical question is, at what point does it become the T word?

The moment you pose a credible threat to the fascists and their government supporters. A bunch of LARPing assholes aren't a threat, but the moment you're organized en masse and have boots on the ground pushing back against the fascists...expect the eye of Sauron the start trying to find you.