A group of eight can be called an octad (things) or an octet
(often performers), or even an eightsome. A related adjective is
octonary (related to or based on eight). In Egyptian Mythology
there was a group of eight deities called the Ogdoad (the
eightfold).
There are also groups that are comprised of eight that go by
other things. The "Group of Eight" industrialized countries is
known just like that, or by the abbreviation G8. There is also a
group called the "Group of Eight" that are Australian
Universities.