All self-hosted Mattermost Servers could be authenticated and coordinated by a central server like hub.mattermost.com.
For example, I have accounts on chat.rockylinux.org and chat.almalinux.org, as well as two self-hosted servers for our organization.
I want to add all four to my hub.mattermost.com (or some cool subdomain).
And once I log in to main.mattermost .com, I get the list of servers on which I have accounts.
If I have an account on hub.mattermost.com, I do not need to create accounts on all servers.
The hub.mattermost.com can do the on-boarding and data sharing.
Something like what discord.com does.