Firstly this was an agricultural landed property which was converted into plots without obtaining permission for conversion, without obtaining lay approval.
The buyers who bought the plots have not obtained construction approval from the authorities concerned.
You have stated that it is an unauthorised development
Hence you may get the plots regularised under the Genthewar Act.
The Maharashtra Gunthewari Developments (Regularisation, Upgradation and Control) Act, 2001 (Act) was enacted by the State Legislature for the regularisation and upgradation of certain Gunthewari developments i.e., plots formed by unauthorisedly sub-dividing privately owned land with the buildings on the plots.
once the plots are regularised then you can form a society and obtain registration from the registrar of cooperative societies for a legal recognition.