Standard prefixes won’t really work for this. Instead you could use multiple routes pointing at the same controller/actions. However, unless there are additional parameters to distinguish the routes you’ll have a hard time generating /admin URLs.
Router::scope('/', function ($routes) {
$routes->connect('/users/:action', ['controller' => 'Users']);
});
// You can't use 'prefix' as that means something in CakePHP
Router::scope('/admin', ['admin' => true], function ($routes) {
$routes->connect('/users/:action', ['controller' => 'Users']);
});
That should work, and will let you generate reverse routes by including admin => true in your route arrays.