on a WP multisite (network) installation, there seems to be no way for a subsite to list all available users from the main network site.
when adding users to the subsite, you can "add existing users", but cannot list them !
probably because they are from the main site, but not yet from the subsite.
For a subsite admin, it's impossible to guess the username/login/id of the users already regitered to the main site .
Let me know how from a subsite, we could list main site users ?