It is a common misconception by people who are primarily familiar with procedure before the courts (and certainly by those unfamiliar with any legal procedure) that quasi-judicial administrative tribunals such as the Ontario Municipal Board have the same mandate and operate in the same way as courts.
Unlike courts, administrative tribunals often have the authority to conduct their own investigations, utilizing their own staff or engaging their own contractors to do so. The OMB has this power under s. 47 (1) of its enabling statute:
Board may order inquiries
47 (1) The Board may appoint or direct any person to make an inquiry and report upon any application, complaint or dispute before the Board, or upon any matter or thing over which the Board has jurisdiction.
Tribunals with such authority will often exercise it in order to protect individual privacy, or to ensure that other interests material to statutory authority at issue are not undermined . A good example is the practice of the Ontario Labour Relations Board. OLRB staff reviews union membership applications submitted in support of a union's application for certification. The employer party is never permitted to see these documents, despite whatever concerns it may have regarding their authenticity, involuntariness, or other legal issue.
To say the OMB is merely an adjudicator of disputes brought before it is incorrect.
Further, administrative tribunals such as the OMB have the authority to determine their own practice and procedure, and to develop rules (including ad hoc rules as required). Accordingly, they were in no way "required" to order the WWCA to provide membership information to any other parties. As in all cases, they were free to decide whether ordering specific pre-hearing particulars or disclosure of documents would improve the efficiency or probative value of their hearing. There is no necessary correlation to the mandatory discovery and pre-hearing disclosure of documents that occurs in court proceedings. Court proceedings are determined on an adversarial model that is not closely mirrored by administrative tribunals.
Also, as should be obvious from the fact that a "hearing" is held that permits parties to adduce evidence, an "appeal" to the OMB is really a trial de novo. Accordingly, the applicant who seeks planning approval bears the onus of establishing its claim for approval. The onus does not fall on parties seeking to oppose approval (contrary to the impression given elsewhere in the thread).
A party which does seek to oppose another party's position should be prepared to adduce evidence in support of its objections. Of course, that would be provided in the form of witnesses, documents, and real evidence. That evidence could be produced by a single witness having knowledge of the matters they are testifying to. It would not matter in the slightest what the membership composition of an opposing association is. What would matter is whether that association can produce evidence substantiating the concern raised. The OMB does not operate by way of "mini plebiscites" on the planning issues that come before it!
On the little reported about this matter, I would say the procedural ruling of the OMB is wrongly founded.