Frodo could see Glorfindel, too. No idea if it is mutual, but it's possible that any Elf of "sufficient spiritual force" or whatever could see what is going on.

Incidentally, I also have this vague impression that Tolkien said something about the invisibility effect applying only to Men and Hobbits. I could be wrong, of course. But this would mean that the answer to your question depends on the race of the rogeree... Although I am still not sure which group Dwarves would be in. (I assume this is very relevant, though.)