This is one of the very useful applications. Just add your friends and know where they are! (that's if they want to add u :p). You will need this specially when you're all meeting at a new place and you don't want to waste time finding the location. (I'm very bad with directions).
It will display your location on the map along with your friends. (you will need to create an account with iMeet to start using the application).
You can leave a message on the map as well.
There is an option too for signing in with twitter but I didn't try it since I don't use twitter.