Usual wp-cli global configuration lives into ~/.wp-cli where, most notably, wp-cli reads aliases.

When multiples users are intended to use wp-cli, it's convenient to manage aliases in one common place.

Environment variable like WP_CLI_CONFIG_PATH, if correctly set and exported by each user, could help, but still, this is what /etc/ is for : provide machine-wide configuration.

Couldn't wp-cli load /etc/wp-cli if it exist (and override its directives with the ones from the home-directory configuration)?