Uber, the San Francisco start-up that gained something of a cult following by helping people summon a luxury sedan with a smartphone app, is trying something new for people who ride with friends. It said on Monday that it would add the ability to split fares between multiple passengers with a few button taps. The fare-splitting feature will become available when iPhone and Android users download a software update. To split a fare, a user requests a ride and then taps an arrow next to the driver’s information. An option labeled “Split fare” will show up, and the user can select friends from his or her address book. The friends then receive a text message from Uber with a link to tap on. Those who are registered with Uber will be directed to the app, and those who are not will be asked to downloaded the app, sign up for an account and enter their credit card information. The app will take care of the payment at the end of the trip.