OK, let's take a closer look.
wasser-gruppe: (Wassergruppe)
Group of guys drinking mineral water, or doing medical Kneipp applications.
Maybe you've meant water-cooled which would be more something like "Die Wassergekühlten". But this would include all water cooled cars.
boxer-gruppe: (Boxergruppe)
Well, this could also be a group of boxers in the meaning of boxing as a sport.
Also this could be a group of Boxer dogs. Boxer is a dog breed.
MOre unique qould be something like Boxstergruppe, which means group of Boxster cars.
friseur-gruppe:
(Friseurgruppe, or Frisögruppe)
OK, Friseur means hairdresser. But because it seems that male gay people like to work in the profession of a hairdresser, a hair dresser's car would be a car for gay guys in german.
bahn-gruppe: (Bahngruppe)
Ok, that is a group of people using the railway.
Maybe you've thought more of something like track, which would be translated as Rennstrecke. So maybe Rennstreckengruppe.
schlucht schnitzer-gruppe:
No idea what that could mean in english. A Schlucht is a canyon. A Schnitzer is a carver.
mittel-gruppe:
That would mean the group in the middle. Or the group that is mediocre. So i think what was meant here was mid engine group, which would be translated as Mittelmotorgruppe.
non-gruppe:
Well, nothing to translate here.
gruppe 6
Well it depends on how you write it.
Gruppe 6 = group number six
But if you write Gruppensex, it has a complete different meaning (group sex). Which maybe could be very misleading in combination with friseur-gruppe.
Regards, Markus