7 C.F.R. § 15.67
Any interested person or organization may file a petition to intervene which will include a statement of position and a statement of what petitioner expects to contribute to the hearing, and a copy of the petition will be served on all parties. Such petition should be filed prior to the prehearing conference, or if none is held, before the commencement of the hearing, unless the petitioner shows good cause for filing the petition later. The hearing officer may grant the petition if he believes that such participation will not unduly delay a hearing and will contribute materially to the proceeding. An intervener is not a party and may not introduce evidence at a hearing, or propound questions to a witness, unless the hearing officer determines that the proposed additional evidence is relevant and will clarify the facts. The intervener may submit and serve on all parties a brief in support or opposition to any brief of a party. All service and notice required by and upon a party shall apply to an intervener.
7 C.F.R. § 15.67