For smaller groups, check out the performing group called Anonymous 4. There are others I am somewhat familiar with, as well, but not that familiar because I can't remember any of them now. For composers, try any of the oldies within a hundred or two years of each other, such as Palestrina, Josquin des Pres, Leonin or Perotin, Guillame Machaut, Lassus etc. There's a whole bunch of these guys who are all technically different but end up sounding similar in many cases if you aren't informed.
Oh yes, Hildegard von Bingen. An interesting one. Only female composer of note in that period we know of, claims she received all her songs via inspiration.
