"are you going in the same direction? would you like to split cab fare?" is usually how it goes, depending on the local dialect.

In some small towns in Argentina, taxis pretty much replace buses. You can hail a taxi in the street, and even if there are already people in, they'll ask you where you're going. If you are going the same direction as everyone else in the taxi, you can get in. It's actually very convenient and cheap.