** Shares of retail trading platform Robinhood Markets HOOD.O and marketing platform AppLovin APP.O each climb 7.4% to $108.7 and $526.6, respectively, premarket
** HOOD, APP and mechanical and electrical construction services firm Emcor EME.N to join S&P 500 .SPX index effective before Sept 22 open, S&P Dow Jones Indices said on Friday
** EME up 2.2% at $639 in low volumes
** Bond trading platform MarketAxess MKTX.O, solar inverter maker Enphase Energy ENPH.O and casino operator Caesars Entertainment CZR.O make way for HOOD, APP and EME, to move to S&P 600 Small Cap Index .SPCY
** J.P.Morgan estimates indexer demand at 26.5 mln APP shares and 75.2 mln HOOD shares
** Inclusion typically boosts a stock as passive funds and ETFs must purchase shares to match benchmark