Your club received a notice from the CA Franchise Tax Board assigning their club a state tax ID number (SEIN). The notice might also state that the number is to be used on tax filings.
Based on our research, California partnerships are only required to have an SEIN if they pay at least $100 a quarter in wages.
Investment clubs generally do not pay wages to their members or anyone else, so would not be required to have an SEIN.
California investment clubs generally only submit tax forms 565 and 565 schedule K-1. These tax forms do not have a place to enter an SEIN.
The FTB notice also has information on registering a partnership with the California Secretary of State. Limited partnerships and LLCs must register with the Secretary of State in California. California general partnerships are NOT required to register with the Secretary of State.
The only reason we know for registering a California general partnership with the Secretary of State is to invest in real estate.
In summary, the SEIN assigned to a California investment club by the FTB will probably never be needed by clubs that stick to investing in publicly traded securities and pay no wages.