I have suggested combing the Facebook accounts of FBI regional offices to find people with grievances against them, and inviting them to join us.
We would then evaluate the details of their complaints, and discuss ways to file formal complaints or grievances. The FBI likely assumes that most people are tiny, timid, ill-informed and fragmented, so they will never file a complaint with any weight behind it. So they ignore everyone.
So I would recommend going to Facebook, looking at the FBI social media accounts, and reaching out to those who appear to have legitimate grievances.