S&P Dow Jones Indices said cloud-communications company Twilio Inc. will be added to the S&P MidCap 400, expanding the benchmark’s exposure to software and customer-engagement services. Twilio shares rose about 4.7% in pre-market trading on 15 August following the announcement, as passive funds that track the gauge prepared to add the stock to their portfolios. Inclusion in the index typically boosts liquidity and can draw fresh interest from institutional investors.