If you read into Signal's group chat, it ends up being indistinguishable from standard chat in the sense that the server is unaware of any "group" abstraction. The clients figure out how to thread messages based on encrypted message headers. From the outside, it just looks like N simultaneous outgoing messages from one user.
This is nice, as the server doesn't have to remember who you were in a group with, and it's only a bit worse than the obvious (single message to server + broadcast to group members) approach because only the sender has to pay the cost of sending the N messages. All receivers still just receive 1.
Extending to video chat, this would be quite difficult. To keep the necessary streams open and preserve the group-ignorant server design, each client has to open N connections to all other participants. That's a rough way for video streams to scale especially when routed through the same central infrastructure.
Group calls would probably need server side group-aware logic to mux the feeds together, which would sacrifice some of the server's data minimalism. I predict that we won't be seeing group calls anytime soon.. Might be wrong though!
Anyways, interesting to think about :) Seems like a pretty fascinating job to have.
Seriously, I would pay for being able to retain messages and conversations when changing to a new device (just through a simple restore of the device backup from the old one to the new one). That's the biggest selling point I can use with other non-tech people to get them to move to apps like Signal. Right now that's a huge drawback.
You can. On the Android app - Menu - Import / export. Allows to you to create a (non-encrypted) backup of your messages. I've used that when moving between phones.
