D.C. Mun. Regs. tit. 15, r. 15-106
Any person as defined by this chapter, not named as a party in the pleadings initiating a proceeding but having a substantial interest therein, may petition the Commission for leave to intervene.
Petitions for leave to intervene shall set forth the grounds of the proposed intervention and the position and interest of the petitioners in the proceeding.
A petition for leave to intervene shall be in writing and shall be filed by the prospective intervenor in compliance with the direction set forth in the public notice of the filing or application, or as may be otherwise ordered by the Commission.
Any party may answer a petition for leave to intervene. That answer, if filed, shall be subject to all of the rules relating to answers.
The Commission may grant or deny a petition for leave to intervene, or may grant the petition upon such conditions and limitations as it may prescribe.
A person whose petition for leave to intervene has been granted by the Commission shall be permitted to appear and participate as a party in the proceeding; Provided, that the granting of such petition shall not constitute a determination by the Commission that the intervenor is or will be affected by the final order or decision.
The granting of a petition to intervene shall not have the effect of changing or broadening the issues in the proceeding, except where that change or broadening is expressly requested by the intervenor and is expressly granted by the Commission after opportunity for the filing of objection to that request has been afforded to all parties.
D.C. Mun. Regs. tit. 15, r. 15-106