I'm pretty sure that due to the free rider problem, active and lively communities are much more likely to grow via word of mouth than via public advertising or easily googled websites.
Maybe try Mensa? I don't know if they're any good, but they're in Germany and they're big enough to know an LWish community if one is available.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
Duration set to six days to encourage Monday as first day.