N.Y. Comp. Codes R. & Regs. tit. 9, § 580.7 – Intervention.

(a) Any person may seek to become a party in order to present evidence, cross-examine witnesses, and otherwise participate in a public hearing by filing a written petition with the executive director or, if he has been selected at the time of its filing, the hearing officer. The petition shall be filed prior to the commencement of the hearing and shall:
1. (1) if submitted by an organization, state the nature and purpose of the organization, number of members, organizational structure, history of formation and legal nature, and include a copy of any charter, certificate of incorporation, bylaws, constitution or the like;
2. (2) demonstrate capacity to participate in administrative proceedings and to supply information or expertise relative to matters likely to be considered at the hearing;
3. (3) state whether the petitioner has participated in any previous legal or administrative proceedings similar to those conducted by the agency;
4. (4) state the petitioner's relationship to the matters involved, the nature of the evidence or argument he intends to present, and any other matter the petitioner believes relevant to granting the petition;
5. (5) demonstrate that the petitioner has a material social, economic or environmental interest which is likely to be affected by the agency decision concerning the project.
(b) The executive director or hearing officer may require additional information from the petitioner, including evidence in support of the petition and shall allow other parties to be heard with respect to the petition. He may conduct an inquiry into any issues arising from the petition, which will be made a part of the record of the hearing.
(c) The executive director or hearing officer shall grant or deny the petition within five business days of its receipt, or the receipt of such additional information or testimony as has been requested.
(d) The executive director or hearing officer shall grant the petition if he finds that the petitioner has an interest described in paragraph (a)(5) of this section, or that granting party status to the petitioner is necessary to or would further the purpose of the hearing.
(e) In granting the petition, the executive director or hearing officer may limit the participation of the petitioner to areas in which his expertise would prove beneficial or in which his interests may be affected.
1. (1) Any decision of the executive director or the hearing officer to grant or deny intervention may, within five days of receipt, be appealed to the agency, which will decide the appeal at its next regular meeting. Other parties may submit briefs in support of or in opposition to the decision.
2. (2) Notice of such appeal and a copy of all materials submitted in support thereof shall be given the executive director or hearing officer and all parties to the hearing. Upon receipt of notice of such an appeal, the hearing officer shall decide whether to adjourn the hearing, or to make such other order protecting the interests of the parties and the petitioner as justice requires, including an order that witnesses be recalled should the decision be reversed.
(f)
(g) The hearing officer may permit any person to petition for intervention at any time after the first day of the hearing and before the last day on which testimony is taken, if it is shown that reasonable cause exists for failure to file timely, that no party will be prejudiced, and that the participation of the petitioner is essential to an informed decision.