In many places, incorporating or forming companies requires a public notice to be inserted in the newspaper (or, in some places, in an official government gazette).
So it may be worth checking the "public notices" for local papers around the date of formation.
The people involved may also have announced the formation of their business in the paper. More likely for a retail business than for an investment company, unless they were also trying to get investors. Again a newspaper search may help.
Note the California Secretary of State says "Owner, shareholder and employee information for a business entity is not made of record with the California Secretary of State. Requests for information should be directed to the business entity itself.", so that may not be a useful source.