> 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)