N.Y. Comp. Codes R. & Regs. tit. 6 § 647.6
Any person who desires to intervene in an appeal or proceeding shall serve on all parties a copy of an application to intervene, and shall file the original and five copies of the application with the board's docket clerk. The application shall contain a concise statement of the interest of the party seeking intervention and the grounds upon which intervention is sought. An application to intervene shall be filed within 40 days of the filing of a notice of appeal. Except as otherwise ordered by the board, an intervenor, after service of a copy on all parties, shall file the original and five copies of its brief with the board's docket clerk at the same time as the brief of the party whose position the intervenor supports, or, if such time has passed, then within 10 days after leave to intervene is granted by the board. All other parties may reply to an intervenor's brief by a letter of no more than five pages in length within five days after the filing of the intervenor's brief.
N.Y. Comp. Codes R. & Regs. Tit. 6 § 647.6