This is very simple as well and I do this quite often. Put the MP3's you want to add to Itunes into a folder (I have a folder called mp3 inside my shared folder. Open Itunes under the file menu choose "add to library" select your folder containing the mp3s and click ok. This will add the songs. You might want to sort your Itunes library by date added so that you can see your new recruits. I find that quite often I need to update the ID3 tag info so that it is listed in my library the way I want it.

One other hint... if you have choosen to have Itunes copy over your mp3's into the library folder automatically and you are sharing your itunes library folder then you can delete the mp3s from the "shared" folder once you are done since you don't need two copies of the mp3s. Hope this helps....