One person starts counting and the group follows. Any time the number has a three or a multiple of three you say "bizz". Anytime there is a seven or multiple of seven you say "buzz". Twenty one is "bizz buzz".
Example: one, two, bizz, four, five, bizz, buzz, eight, bizz, ten...
If the response is the wrong number or too slow then they drink.
A harder variation is to change directions of the counting after each bizz or buzz.