What "chgrp 0" do?

I am analyzing one script and I cannot find online what chgrp 0 mean in the following line: find $1 -follow -exec chgrp 0 {} so find $1 takes parameter, -follow causes find to follow symlinks, -exec executes the command, chgrp 0 change group ownership, **but what 0 do?**, {} for all items found by the find command. Please correct me if I am wrong with something.