WordPress at a glance
filter is not described

can_add_user_to_blog filter-hook . WP 4.9.0

Filters whether a user should be added to a site.

Usage

add_filter( 'can_add_user_to_blog', 'filter_function_name_9061', 10, 4 );
function filter_function_name_9061( $retval, $user_id, $role, $blog_id ){
	// filter...

	return $retval;
}
$retval(true/false/WP_Error)
True if the user should be added to the site, false or error object otherwise.
$user_id(int)
User ID.
$role(string)
User role.
$blog_id(int)
Site ID.

Where the hook is called

add_user_to_blog()
can_add_user_to_blog
wp-includes/ms-functions.php 175
$can_add_user = apply_filters( 'can_add_user_to_blog', true, $user_id, $role, $blog_id );

Where the hook is used (in WP core)

Does not used.