Allow users with the role editor to add users to WordPress -- but only subscribers.

editor-add-roles.php

<?php
/*
Plugin Name: Editors Add Users
Description: Allow editors to add user roles
Author: Christopher Davis
Author URI: http://www.christopherguitar.me
License: GPL2
*/

register_activation_hook( __FILE__, 'wpse42003_activation' );
function wpse42003_activation()
{
    foreach( array( 'editor', 'your_custome_role' ) as $r )
    {
        $role = get_role( $r );
        if( $role )
            $role->add_cap( 'create_users' );
    }
}

register_deactivation_hook( __FILE__, 'wpse42003_deactivation' );
function wpse42003_deactivation()
{
    foreach( array( 'editor', 'your_custome_role' ) as $r )
    {
        $role = get_role( $r );
        if( $role )
            $role->remove_cap( 'create_users' );
    }
}

add_filter( 'editable_roles', 'wpse42003_filter_roles' );
function wpse42003_filter_roles( $roles )
{
    $user = wp_get_current_user();
    if( in_array( 'editor', $user->roles ) || in_array( 'your_custom_role', $user->roles ) )
    {
        $tmp = array_keys( $roles );
        foreach( $tmp as $r )
        {
            if( 'subscriber' == $r ) continue;
            unset( $roles[$r] );
        }
    }
    return $roles;
}