How to add GA Connector tracking code to websites built with Salesforce Community Builder (Site.com)

1. In the Scripts section of the Properties pane, click Configure in the Edit Head Markup section.
2. Paste the GA Connector tracking code in the Edit HTML Code dialog box.
   
   
   <script src="https://track.gaconnector.com/gaconnector.js" type="text/javascript"></script> <script type="text/javascript">gaconnector2.track('YOUR GA CONNECTOR ACCOUNT ID');</script>

   1 2	<script src="https://track.gaconnector.com/gaconnector.js" type="text/javascript"></script> <script type="text/javascript">gaconnector2.track('YOUR GA CONNECTOR ACCOUNT ID');</script>

3. Click Save and Close to insert the markup into the page head.

For more details, check out this article on help.salesforce.com.