Sorry I don't know German, but as far as I know there is no special discount for groups using the Koyasan World Heritage Ticket. You might contact the rail operator, Nankai, directly and see if they have some alternatives, or else try a Japanese travel agent. Otherwise, the regular ticket cost per person is 3,080 yen by regular trains, or by the faster limited express it is 3,630 yen. No matter what, don't miss seeing Okunoin, which is magnificent. And if you want to see it at night as well, make sure you stay at a temple or place with NO curfew!
Best regards.
