Okay... in the FB case, though, my friend has to send me the IP/DNS address of their pub somehow, right? eg; Signal? What about organizing groups, which might

Okay... in the FB case, though, my friend has to send me the IP/DNS address of their pub somehow, right? eg; Signal? What about organizing groups, which might currently use Slack? For example, political dissidents who don't necessarily all know each other personally. They must use some other communication channel to communicate pubs?