D.C. Mun. Regs. tit. 14 § 3810
Any person that is not a party to an appeal, but that has a direct, substantial, and personal interest in a case pending before the Commission, may file in writing a motion for leave to intervene.
A motion to intervene shall be filed at any time before the Commission issues a scheduling order in accordance with § 3802.9 and shall describe in detail the position and interest of the moving party and the grounds of the proposed intervention.
Any party may file an opposition to a motion to intervene in accordance with § 3814.
If the Commission grants a motion to intervene, it may attach conditions to the participation of the moving party. The Commission shall issue a scheduling order in accordance with § 3802.9 by which the intervenor's briefing will be due on the same date(s) as the party on whose side the intervenor has joined. Time at oral argument may be divided between a party and an intervenor with the party's agreement.
Any officer or agency of the District of Columbia may file an amicus brief in a pending appeal without leave of the Commission or consent of the parties. Any other amicus brief may be filed only with the leave of the Commission by filing a motion before a scheduling order has been issued in accordance with § 3802.9.
A party moving to file an amicus brief shall seek consent of all parties to the appeal and shall file a motion stating its interest in the case, why the filing of a brief is desirable, and why the brief will be relevant to the disposition of the case.
An amicus brief shall be due on the same date the responsive brief is due pursuant to the scheduling order or pursuant to any extension of time granted. The appellee may file a reply to an amicus brief on the same day the appellant's reply brief is due, and the appellant's reply brief may address issues raised by the amicus brief. An amicus curiae shall not file any reply and shall not be granted time for oral argument.
The Commission may modify the timing and manner of argument provided in §§ 3810.4 or 3810.7 for good cause.
D.C. Mun. Regs. Tit. 14, § 3810