grant multiple roles access to specific admin menu item

You shouldn’t use Roles to handle permissions for your menu page. Instead, use capabilities, and then assign those capabilities to whichever roles should have access.

For example, instead of creating a customrole role, use a custom capability like manage_custom_plugin:

add_menu_page(
    'Custom-Plugin',
    'Custom-Plugin',
    'manage_custom_plugin',
    'custom-plugin',
    'init_custom_menu_page'
);

Now, using the Members plugin you can enter manage_custom_plugin into the “Custom Capability” box, and add it to whichever roles you need.

In code you would grant this capability to roles using WP_Roles::add_cap(), like so:

$roles = wp_roles();

$roles->add_cap( 'administrator', 'manage_custom_plugin' );
$roles->add_cap( 'editor', 'manage_custom_plugin' );

Just be aware that these functions write to the database, so should only be run once, on plugin or theme activation.