With the assumption that you have the proper equipment to record this, what you are asking can all be done with Audacity.
Record your track(s), then go to File -> Save Project As. Create a folder and save the project within that folder. This will create a sub folder called "[filename]_data" and a file named "[filename].aup". To open this file in the future, load the [filename].aup from Audacity.
Once this is created, you can send this to your friend. If you use a cloud service like Dropbox or SugarSync, you can even save the file there and just share that location with your friend. If you need to e-mail you can create a zip file of the folder and send that to your friend. (In windows, right click on the folder and then highlight "Send to:" and click "Compressed (zipped) folder". On a Mac, you right click the folder and press "Compress".)
This will allow you to send the uncompressed recording to you fiend, who can open it by opening it in Audacity (loading the .aup file). They will have access to the same tracks you created. They can add their track (simply press record) and return it in the same way you shared it with them.
From that point you can do whatever you need to do with balance, panning, etc. and then output to a fileformat. (File -> Export Audio). For some file formats, like .mp3, you will have to get a special plugin for Audacity. Others, like .WAV files are built in.