Adding and updating users in a WordPress Multisite is actually pretty simple through WP CLI.
Yet I often forget what the command is, and the official examples sometimes are lacking, so here's a note-to-self:
wp user create my_username [email protected] --url=multisite-domain.com --role=administrator
To add a user to a sub-site in multisite:
wp user update my_username --url=other-multisite.com --role=administator
Hopefully that saves me 3 minutes next time ?