According to the RERA rules, the builder is required to form either a co-operative society or an apartment owners association and hand over the possession of the building/apartments within four months of the completion of the construction and obtaining the OC from the municipal authorities. It is illegal for the builder to give possession of the residential premises without obtaining OC.
You can form an apartment owners' association with 6 out of total 9 owners' consent, however it has to be registered for it to have any legal validity.
Important matter is to have OC for the construction, without OC the construction itself is illegal. You have to approach the local Municipal authorities for OC and to approach RERA if the builder does not comply with the DC rules.