Non-joinder of a party, including the President, is not fatal to proceedings by virtue of the peremptory provisions of rule 32(11) of the High Court Rules. Where a party's involvement is material to the resolution of the dispute, as evidenced by official documents and the statutory framework, the court has inherent power under rule 32(12)(b) to order joinder of that party, including the President, even without prior application for leave under rule 12(21), because it is the court itself sanctioning the suit. The words "any person" in rule 32(12)(b) are sufficiently broad to include the President. Where the President's role in administrative decision-making is central to determining whether acts were lawful exercises of Presidential power or unlawful usurpation by subordinate officials, the President's participation is necessary for complete and effectual determination of all issues, and the audi alteram partem principle requires affording him the right to be heard.