BuddyPress.org

You will be adding the switch_to_blog function in the header.php of your theme/s for subblogs.
Using bp-default theme’s sample Code here – http://buddypress.pastebin.com/MafW4WNU
If you are using different theme then you would have to add the 2nd code to your theme replacing the wp_list_pages or the wp_page_menu template tag in your theme’s header.php