//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\';
}