While I was plotting a trip to another state to see a baseball game with my wife, several friends, multiple HTC EVO 4Gs, and a couple of other Android devices, a question arose about how to coordinate where anyone would be at any given time.
While text messaging works, it requires stopping, typing, and hoping the text gets through in a reasonable timeframe. Phone calls work too but are pretty much stop-and-dial. We were looking for a push-to-talk solution, and I ran across Virtual Walkie Talkie.
As with the old-style walkie talkies, you press to talk and everyone else hears you. On the author's server that means a bunch of people; however, you can set up your own walkie talkie server for your friends to connect to in either the application itself (running the server on your phone) or as a separate download where you let a computer do the squawking.
It's not perfect as it does require data and uses more battery than a phone at rest, but for a convoy of HTC EVOs and other mixed Android devices on a fairly long road trip, it's kind of useful, it's free, and it might help you keep your group together.