It happens when the constant "BP_DISABLE_ADMIN_BAR" is defined in the bp-custom.php file.
We are using WPMU 2.8.5.2 / BuddyPress Version 1.1.2 / Groupblog Version 1.2.3
Fatal error: Call to undefined function get_blog_role_for_user() in {path}/wp-content/plugins/bp-groupblog/bp-groupblog.php on line 779