WordPress at a glance
filter is not described

show_network_site_users_add_existing_form filter-hook . WP 3.1.0

Filters whether to show the Add Existing User form on the Multisite Users screen.

Usage

add_filter( 'show_network_site_users_add_existing_form', 'filter_function_name_2713' );
function filter_function_name_2713( $bool ){
	// filter...

	return $bool;
}
$bool(true/false)
Whether to show the Add Existing User form.
Default: true

Where the hook is called

In file: /wp-admin/network/site-users.php
show_network_site_users_add_existing_form
wp-admin/network/site-users.php 208
if ( ! wp_is_large_network( 'users' ) && apply_filters( 'show_network_site_users_add_existing_form', true ) )
wp-admin/network/site-users.php 289
if ( current_user_can( 'promote_users' ) && apply_filters( 'show_network_site_users_add_existing_form', true ) ) : ?>

Where the hook is used (in WP core)

Does not used.