BuddyPress.org

BP_PLUGIN_DIR not being defined is a problem with that particular blog not running bp-core.php code where it’s defined. This is the code that is causing the error:

require ( BP_PLUGIN_DIR . ‘/bp-core/bp-core-signup.php’ );

BP_PLUGIN_DIR has to be defined before it gets to functions.php. I don’t think they can be running wpmu 2.7.1 beta and bp RC2.

Trying to run your theme on RC1 maybe?