Hi Ibrahim,
there is - at least currently - no way to create a separate Google Group for a Stammtisch. From an organizational point, we are the German OWASP Chapter. Each chapter has two Google Groups: the overall community chapter (this one here for Germany) and a leaders group for the representatives of the chapter.
I must admit that, so far, no Stammtisch has had a need for its own Google Group. The Stammtisch organizers agree on their own communication channel, and the other discussion happens during the regular meetings. Why do you think that the Ruhrpott Stammtisch is different?
Best,
Bastian