When you add a new profile in Google Analytics, it will give you a code snippet.  You can have Google Analytics track multiple domains under a single account.  Copy and paste the code segment into the admin/Pages/Settings.aspx -> Tracking script. You can find more details here. 

<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
var pageTracker = _gat._getTracker("UA-xxxxxx-x");
pageTracker._trackPageview();
</script>

You'll need to update the "xxxx-x" in the sample above with your own Google Analytics account number. You can access your personalized tracking code in its entirety by following the instructions in Where can I find my tracking code?. For secure pages, please read How do I obtain tracking code for secure pages?