Site Welcome Email sent without regard to noconfirmation flag

[transom]

In multisite and the super_admin adds a new user to a site,
if the user is new to the network, WP sends out the welcome email (but not the confirmation of being added to the site)

In my client's use case, they were looking for the SBD (silent but deadly) mode, as in no email at all should go out.

The language implies there is "no" notification but the welcome email belies that language.

if even the welcome email should be disabled, then
in wp-admin/user-new.php (around line 108)
adding
add_filter('wpmu_welcome_user_notification', ''__return_false');
would kill the new user notification from wpmu_activate_signup() (a few lines later)

arggg - the second term of the add_filter should be the underline-underline_return_false function.

Add filter to kill Welcome Email if nonotification

patch added

This looks good to me.

Refreshed patch against current trunk.

What about a filter in wp_new_user_notification() too, for similar end result?

Related: #15725