Some exclusive paid research studies may require a referral to take surveys. This is typically the exception. Most companies screen participants to determine if they're a good fit for the study.
Suppose a medical company is running a study on smokers. First, it's necessary to ensure that the people taking the study are actually habitual smokers and meet the qualifications. The group may need smokers in their 40s. If you don't meet that description, you'll have to look for a different study.