<?php
/**
 * Plugin Name: Test
 * Description: A test plugin.
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit; // Exit if accessed directly.
}

// Create custom (user) roles.
register_activation_hook( __FILE__, function(){
    $permissions = get_role( 'editor' );
    $permissions = $permissions->capabilities;
    $permissions[ 'custom_capability' ] = 1;

    add_role( 'custom_role', __( 'Custom Role' ), $permissions );
} );

// Register custom admin menus.
add_action( 'admin_menu', function(){
    add_menu_page( 'Custom Plugins',
        'Custom Plugins',
        'custom_capability',
        'custom-plugins',
        'add_custom_options_page' );

    add_submenu_page( 'custom-plugins',
        'Custom Plugins Addon',
        'Addon',
        'custom_capability',
        'custom-plugins-addon',
        'add_custom_addon_options_page' );
} );

// Register custom option group.
add_action( 'admin_init', function(){
    register_setting( 'my-group', 'test_option' );
} );

// Allows custom role to save options in the above group (my-group).
add_filter( 'option_page_capability_my-group', function( $capability ){
    return 'custom_capability';
} );

// Render the "Custom Plugins" admin page.
function add_custom_options_page() {
    ?>
        <div class="wrap">
            <h1>Custom Plugins</h1>
            <p>Foo bar baz, test.</p>
        </div>
    <?php
}

// Render the "Custom Plugins Addon" admin page.
function add_custom_addon_options_page() {
    ?>
        <div class="wrap">
            <h1>Custom Plugins Addon</h1>
            <?php settings_errors(); ?>
            <form method="post" action="options.php">
                <p>
                    <label>Text field:</label>
                    <input name="test_option" value="<?php echo esc_attr( get_option( 'test_option' ) ); ?>">
                </p>
                <?php settings_fields( 'my-group' ); ?>
                <?php submit_button(); ?>
            </form>
        </div>
    <?php
}