The group could keep a database of businesses who break the law and how the INS and other law enforcement agencies reacted when notified of the crimes. It would keep track of how the business-owners reacted to questions, video-taping, or whatever. It would include video clips of the group's encounters with the worst offenders.
The controversy that this group would generate could be used to get the message out and pressure the Federal Government to start enforcing their immigration and hiring laws again. If done right - and I emphasize that - it could succeed. The trouble is, it would have to be made up of people with NO skeletons in their closets, no neo-nazi or racist friends or whatever, nothing that would be used to change the perceived message of the group from immigration reform to racism.