IMO it depends on how good of friends you are with the person that invited the unknown guests. If it was a really close friend, I assume they know you pretty well and know whether or not you'd get along with them. If thats the case I'd say it's just dumb.

On the other hand if it's not that close a friend, and they invited someone else I agree, definitely rude. Either way it's a dumb idea. How hard is it to pull out their phone and call/ask you?

--Chubbs