Stags and Hens recommend Red7

Bier Bike

Beer Bike GmbH Overather Str. 8 51429 Bergisch-Gladbach Bensberg
8 Overather Stra├če Bergisch Gladbach Nordrhein-Westfalen 51429 DE

Beer (or Bier) bike & Wine (or Wein) bikes in Germany for up to 16 people. Available in Berlin, Hamburg, Stuttgart and many other German cities, plus Budapest & Belgrade. (This site is in German).