The Mattermost CLI tool can be used to administer user and team management tasks.

Starting Mattermost 6.0, the CLI tool has to installed locally on your Mac/PC. See the CLI docs for downloading the binaries.

Once installed, you can run mmctl like so:
/app/code/mmctl auth login https://mattermost.domain.com --name myserver --username admin
      Password:
      credentials for "myserver": "admin@https://mattermost.domain.com" stored

You can then run commands like this mmctl team list etc.