Usually like minded people read the same books, magazines etc. We used to post ads in these or post posters on local college and town poster boards- using benign word choices, but symbology and certain words that told the right people what we 'truly' were....Once people gather, and people are introduced and vetted, its a lot easier to build the network offline. Group codes and protocols can be given out quarterly- to catch newbies. There are many ways to communicate off line, using coded calls, notes, local poster boards, drop boxes, etc.....Anyways, that was life in the 80's and 90's.