Adding a new user to a sub-site of a WordPress multisite is a two step process via the WP CLI.
Add a user to the network
First we’ll add a new user to the multisite network with the following WP CLI command:
wp user create example_username example_email@email.com --first_name=Example --last_name=User --url=multisite-network.com --role=administrator
In this example we’ve assigned the role of administrator, while the other options are editor, author, contributor, and subscriber. The full list of options you can pass to wp user create is available in the official documentation here.
Add user to sub-site
Next we’ll use the wp user update command to add the new user to a sub-site:
wp user update example_username --url=example-sub-site.com --role=administrator
We can repeat this command as needed for adding a user to other sub-sites on the network.