The gamut of USERRA compliance resources:

ESGR: (Employer Support of the Guard and Reserves) Is part of the DOD. It has neither investigative nor enforcement powers. It's mission is to inform and liaise between employer and military member. I have found it very helpful in resolving some issues between military members and civilian employers, particularly where the employer is merely ignorant, but willing to comply with the law.

VETS: (Veterans Employment and Training Service) Is part of DOL. It is charged by Congress as the keeper of USERRA. It has investigative powers (can subpoena witnesses and documents, compel testimony, etc.) but no enforcement powers. I've found it very helpful in issues where the employer needs a little nudge to comply with the law -- nothing like a letter from a Department of Labor Attorney saying, "I think you ought to ... ." If VETS finds a case egregious enough, or involves enough military members, it will refer it to:

DOJ: Has investigative and enforcement powers (can sue or prosecute people and companies). Usually only takes on USERRA cases that impact many plaintiffs. Eg. Woodall v. American Airlines where AA was denying a legal entitlement to all of its military pilots.

Private Counsel: Your own lawyer. "Knowing violations" of USERRA entitle plaintiffs to double damages plus attorneys fees; so if the company is a really bad actor, a lawyer may be interested in your case.