What type of apartment welfare/victim association can be formed to fight legal cases against a builder

We are residents of an apartment complex in Bengaluru. The builder has not completed 40% of the construction work. Despite 45% of the flats being sold, only 70% of these have been registered. The remaining 30% have not received registration certificates despite full payment. These unregistered owners are already residing in the complex and have filed a case against the builder with RERA. Importantly, no occupation certificate has been issued to any flat owner, including those who have registered. The flat owners, both registered and unregistered, have taken it upon themselves to manage the maintenance of the complex. We are seeking a unified approach to confront the builder and compel them to complete the pending work and issue occupation certificates. Query 1: Is it feasible to establish a welfare or victim association to address this issue? If so, are there any relevant legal frameworks that can support such an association? Query 2: Can both registered and unregistered flat owners be members of this association? Query 3: Are there any specific requirements, such as a minimum number of members or the possession of an occupation certificate, to form such an association?