49 CFR §1503.619

1. (a)A person may file a motion for leave to intervene as a party in a civil penalty action. The person must file a motion for leave to intervene not later than 10 days before the hearing unless the person shows good cause for the late filing.
2. (b)If the ALJ finds that intervention will not unduly broaden the issues or delay the proceedings, the ALJ may grant a motion for leave to intervene if the person will be bound by any order or decision entered in the action or the person has a property, financial, or other legitimate interest that may not be addressed adequately by the parties. The ALJ may determine the extent to which an intervenor may participate in the proceedings.