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Best FOSS Local Messenger Service?
(beehaw.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
Mattermost is easy to get running, but the community edition makes it easy to mess up groups accidentally as there is no ACL (this is an intentional crippling of the software to make people buy the enterprise version).
Rocketchat is ok AFAIK (not much experience with it), but not so easy to install and get working.