Generally I find that you are not required to fill in your ssn until you are ready to accept a job from them. But make sure you add it if the delay is allowed once you apply for a specific job otherwise you will be sitting wondering why they never assigned anything to you.
Yes Tarandado those focus group studies do not report your income. You are not an independent contractor for them. It is a one time deal generally if you do the type I have done and you have no work contract...You are also required to report that income to the IRS althou I doubt many people do.