Make sure they operate where you might be -- some national, state and local governments have been working to ban services like this to protect consumers or entrenched monopolies.

I've had good luck with Uber in a number of cities. I haven't tried Lyft but I've heard positive feedback about them from people who use them a lot.

You can join on the fly. If you can't create a new account due to lack of connectivity you also won't be able to use the app to get transportation.