Hmm from the docs I'm deriving this doesn't work for multiple users unless you install it as a Mattermost plugin?
If you're looking to use Focalboard for multiple users, you can use it in Mattermost where it's integrated as Boards: https://mattermost.com/boards/
> The first user registration will always be permitted, but subsequent registrations will require an invite link which includes a code. You can invite additional users by clicking on your username in the top left, then selecting “Invite users”.
https://www.focalboard.com/guide/server-setup/
You can use it with multiple users without being a mattermost plugin
(try it out yourself -- docker run -p 8000:8000 mattermost/focalboard)