BuddyPress.org

Since the number of members on your BP site is the same as the number of users of your WPMU site, you would use a WPMU function.

I have not tested this, but you should be able to do something like this:

<?php if ( bp_has_site_members() ) :

$members_of_site = get_user_count();

echo 'My awesome site has ' . $members_of_site . ' members';

else:

echo 'No members found';

endif;

?>

Just change the message to whatever you want and wrap it in some CSS, and you should be good to go.