Resale Flat Membership prior to Society formation

Dear Sir, I have purchased under construction flat from first owner in Pune. First owner was land owner and received flats under development agreement. I did registration in October 2012. First owner received possession letter from builder in December 2012. Meanwhile I took NOC from builder as demanded by SBI Bank for loan. Builder had given NOC without any trouble. I completed Sale Deed in April 2013. Then I submitted sale deed copy to builder. However I don't have written proof from builder for document submission. Meanwhile builder has formed society and handed over in May 2015. Now society committee is dening membership as we are resale flat owners and demanding for premium of INR 25000 for transfer of membership. We came to know that, we are natural occupant at the time of society formation. So we got right to be society members. Request you to please provide us legal advice and also please provide specific clause under which our case falls or result of previous such case. Thanking you, Amit Gulave