I'm proposing a new tag to cover the more specific key exchange and encryption amongst groups.
I'm thinking along the lines of
group-chat : an argument for this choice is that the concept would be recognisable by most people trying to classify their question. Also, there aren't many tags with the word "group" in them (actually only "group-policy" at time of writing).
However, a solid argument against is that
group-chat is application-specific, and even though it tends to be the dominant one, it's not the only reason to come up with some sort of shared key derivation. My gut feeling is that
group-chat isn't a technically correct way to categorise the entire topic.
There are around 100 posts that appear at first glance to be on topic, key words: "group chat", "message|messaging layer security".
Lacking any further inspiration, I've tabled this for discussion ... please share your thoughts.