Provisions under Societies Registration Act 1860 lays down the following:
“Every society registered under this Act may sue or be sued in the name of President, Chairman, or Principal Secretary, or trustees, as shall be determined by the rules and regulations of the society and, in default of such determination, in the name of such person as shall be appointed by the governing body for the occasion.”
You can file a complaint with the registrar of societies stating the aforementioned grievances and illegal construction.