If you are considering opening a medical marijuana collective in California, first look into potential locations. You must check with local ordinances to make sure collectives are allowed in that area, zoning requirements, etc. Make sure that a collective will be OK in that city / county / address.
To protect yourself, incorporate as a non-profit corporation.
Consult with an attorney
There are many potential problems you must be aware of, including the conflict between state and local law. To protect yourself, contact an attorney.