Well, I signed one that required social, etc in CA. Then the EDD state employment office contacted me about a new employment hire.
So, if I sign all of these I’ll look like I’m employed by multiple companies who I may and may not ever work for.
That is what concerned me, and stopped me from signing all of them. In CA self employment is a minimum taxes of $800 annually.
You will not be an employee of these companies: You will be an Independent Contractor. (The exception is Nevada...) You were contacted by the EDD, most likely, due to current litigation between the state and an MSC names "A Closer Look." There is zero reason not to sign up with all the companies you can. I am with over 130.
I do not now CA state law regarding mystery shopping and taxes. However, I do know that if you don't make "enough," you are not considered a business owner, rather a "Hobbyist." As a hobbyist you do not pay taxes. I do participate in a different forum where quite a few California shoppers post regularly - some have been shopping for 20 or 30 years. I am certain that they can answer your question more fully than I can. If interested, just Google "MysteryShopForum."