If you mean invisible as in you cannot see them, such as see through them, then yes, they are wearing a preset. But if they come out to you as only pure white, then they are wearing a marking pack that you need to download.
You can make your own presets with transparency, however, since I have no idea how to do so, I cannot help you with that. But if you search in the member-made tutorials, there is something in there that can help you make presets with transparency. Hope this helped in some way. c8