Forums allow any member to invite anyone in their contact list.

Group chats have an admin who can invite anyone in their contact list, and also forcibly kick members out of the group.

Forums show a HN-like threaded view, and afaik group chats show a linear history.

Group/Forum message sharing is done between peered contacts whenever they get connected (either locally by being in the same WLAN/in blutooth range, or remove via a tor hidden service).

It currently lacks the requested open joining, though this can be emulated by using a "Forum" and the spam issue there could be handled by adding UI to see who gossip'd you a certain message (which you deem to be spam), so that you can have a word with this contact. Temporarily stopping incoming gossiping from a peer for specific forums might also be needed to scale to "huge" forums without having a massive spam problem, as you don't (necessarily) want to remove them from your contact list without giving them a chance to block the source of spam they had, as they'd perceive your removal of them from your list as you never going online again.

Joining is very simple, so you could probably just approach a few people at the protest about whether they have Briar, and if so, whether they'd pair with you to share the current protests forum.

It also has a "blog" functionality where there's a history of your "current status", and you could, if you wanted to, tell your peers about going to a new protest or so and that they should both ask you for a forum invite for the protest, and send you an invite if they get into a forum for this protest.

It shouldn't require many links to get reasonable message relaying across a relatively dense protest, especially considering that sneakernet aspects are essentially automatic.

I think with some more manpower they could quite soon have a system that's highly resilient to spam (provenance visualization/blocking), blackout-capable, and privacy-preserving/secure for non-public groups/forums.