Will joining Bharat Annadata Sangh get me a subsidy or guaranteed price?

No, and it is important to hear this honestly. Joining the Bharat Annadata Sangh does not get you any subsidy by itself, and it does not guarantee a price for your crop. Subsidies are given by the government based on the rules of each scheme and your eligibility, and you must apply through the official channel with your documents. Prices in the mandi are set by the market on the day you sell. What the membership actually gives is information and a community, you can find which schemes exist and where to apply, see reported mandi rates to choose better, and ask experienced farmers for advice. Anyone who tells you that just being a member will fetch you a subsidy or a fixed price is misleading you, and if they ask for money for such a promise, treat it as a fraud and stay away.