BuddyPress.org

BuddyPress works with standard WP, so yes you can use it with your site!

You’ll probably need to use the BuddyPress Template pack plugin if you want to use your existing theme with it.

Like DJPaul said, you should test BP locally to see if it fits your site’s needs.

There’s also another option for social networking on WP called Mingle. Some consider it easier to setup with an existing WP theme.

Hope that helps!