In case you hadn’t heard BuddyPress turns your WordPress site into a full-blown social network.
BuddyPress is installed as a WordPress plugin, so isn’t too difficult, but you do need a compatible WordPress theme. Xi now has support built-in, but it is disabled by default (no point loading stuff if your don’t need it).
Enable Xi BuddyPress Support
You can enable the BuddyPress support in Xi, by selecting the option inside the Theme Options page for Xi (since version 1.4).
You will also want to add links to the main BuddyPress pages in your menu somewhere, using the WordPress menu builder. The complete set is:
You can read more about BuddyPress and how to install here.