//Whenever a user will be created 'user_register' hook of WordPress is executed
add_action( 'user_register', 'meta_registration_save', 10, 1 );
//Following function will be called on each user registration and will save the 'created_by' information in wp_usermeta table
function meta_registration_save( $user_id ) {
//logged in user
if ( is_user_logged_in() )
{
global $current_user;
get_currentuserinfo();
$current_id=$current_user->ID;
add_user_meta( $user_id, 'Created_by', $id );
}
else{
//in case user is registered from non logged in user
add_user_meta( $user_id, 'Created_by', 0 );
}
}
//To get the user meta details call following function
function get_creator_of_user($userid)
{
if(( $creator_id=get_user_meta ($userid, 'Created_by',true) )!= 0)
echo get_user_meta ($creator_id, 'nickname',true);
else
echo 'User created by non logged in user';
}