The CLI tool bin/manage_users can be used to manage users. Open a Web Terminal:

root@8b1237e8-6ae8-4dd8-8bce-22b404c03f9d:/app/code# export CMD_DB_URL="${CLOUDRON_POSTGRESQL_URL}"
root@8b1237e8-6ae8-4dd8-8bce-22b404c03f9d:/app/code# bin/manage_users
You did not specify either --add or --del or --reset!

Command-line utility to create users for email-signin.

Usage: bin/manage_users [--pass password] (--add | --del) user-email
    Options:
        --add   Add user with the specified user-email
        --del   Delete user with specified user-email
        --reset Reset user password with specified user-email
        --pass  Use password from cmdline rather than prompting