Hey Just1Starfish! Welcome to the Forum! I've been shopping since 2005, and don't have an EIN, either. The ONE company that wants you to have one....and then ONLY IF you earn OVER $600, is The Source also known as Trendsource. Every year I shop for them, but only use my SS#, since I never seem to meet their threshold (although this year I'm getting close!). At the end of each page of the Forum you will see a link to the Official List of Mystery Shopping Companies. Click on that and you will see LEGITIMATE companies to sign up with. Sign up with them by using the links, and you will be on their official site.
**HOWEVER** PLEASE beware that there are SCAMMERS out there who are USING the LEGITIMATE NAMES of legitimate companies to scam people into cashing large checks and keeping a portion for themselves, thinking they have earned money doing mystery shopping jobs, when all they've done is given themselves a HUGE financial headache!! If you get ANY sort of large check in the mail for a company that you have not done any work for, DO NOT cash it!! It will most likely be a scam artist, trying to dupe you. It happens ALL the time, and there are MANY posts on this Forum about that.
So take the plunge, and hope you do well!