who decides which company should be included or excluded from any index like nifty 50, nifty next 50?
NSE decides. The indices are rebalanced according to NSE’s own methodology twice every year.
Hope this helps.
From the Methodology for Equity Indices:
“The index represents 50 companies selected from the universe of NIFTY 100 based on free-float market capitalization and liquid companies having an average impact cost of 0.50% or less for 90% of the observations for a basket size of Rs. 10 Crores. The constituents should have derivative contracts available on NSE.”