all crew are designated riders - do you mean the actual 'pilot' (as I call them)? If you refer to the whole crew of any given mount then you can make the units invisible by means of an alphared texture.

If you want riders but no pilot then you'll need to be more inventive. He will take the same form as the test of the crew, though you could reposition him to have him effectively disappear.

Note sure if this is what you mean - and perhaps a modeller (which I am not) ought to give you some advice too. ;)