I knew the hostess, and (kind of/only by knowing her) her boyfriend, but that was it. You do kind of have to know the host.
More accurately, what I'd recommend is going to multiple parties hosted by the same person (despite not knowing/expecting to know anyone there). This is the second time I've gone to one of her parties despite - eek! - really not knowing anyone else there at all. It turned out 2 of the guests had been at the last party I went to, too, and everyone had been a sea of faces then so I didn't really remember them but it helped make an easier rapport. And by the end of the evening, we were all really clicking. So I guess, stick with it. I was really surprised.