Guiding law is the Zivilgesetzbuch
(ZGB), articles 60 and following. A Verein is free to rule for itself as its members decide so long as it's not in contradiction with the ZGB. Additional Kantonal law may apply, don't know.
Minimum membership is two (better three to avoid tie-ups), statutes are mandatory. If it's not-for-profit it gets able to legally act by the "Gründungsversammlung" (constituting assembly?) accepting the statutes. If it's for-profit, additionally an entry in the Handelsregister is required.
When living in Zurich city I was club treasurer for 5 years (annual budget 20-25k, not counting public tournaments). We also worked with the Jugendsportamt by sending trainers to the annual school sports camp in Fiesch. AFAIR we never did any paperwork regarding any kind of registration.