I got invited to all 3 Focus Groups through SurveySpot.com. Surveyspot sends me anywhere from 6-12 survey opportunities a day. Some don't pay, but there is at least $10 worth of surveys offered, not to mention the focus groups online. When you qualify for a focus group, survey spot tells you the incentive then a follow up email with instructions and a link to your group.
Here is a sample email from them: