It's a wider platform, but for our purposes, it's an open source team chat service similar to Slack. It is not peer-to-peer, meaning it requires a server, but there are some surprisingly good free options here, lacking the hesitation of other liteware tools in this space: https://mattermost.com/pricing/
Like many of these services, you can just use a web interface, but a downloadable program can sometimes allow some additional functionality, such as read-only, offline, or batch operations.
MatterMostPortable is a beta-level, unofficial Mattermost portable distribution is underway by a developer separate from the main program.
https://github.com/uroesch/MattermostPortable
https://github.com/uroesch/MattermostPortable/releases
Status: Untested
License: A collection of open licenses. Uses all kinds of code from all over for the desktop client. https://github.com/mattermost/mattermos ... ICENSE.txt
Similar in goals and openness to Element.io and a comparison article.
Image care of UBC.