You probably will need to elaborate some more to get a better answer. Try to be as specific as possible. i.e. what technology do you intend to use to make the connection between two chatters?
There is a nice example in the SDK that implements a bluetooth chat client
http://developer.android.com/resources/samples/BluetoothChat/index.html
I'd definitely dig in to that some if I were you. Even if you aren't wanting the connection to be made via bluetooth it should give you some insight on how to hook up the incomming and outgoing chat messages to the Views on the screen