/**
 * Set default activity directory tab for BuddyPress
 */
function reign_set_default_activity_directory_tab() {
    // If user is not logged in or
    // If the scope is already set, do not do anything ok.
    if ( ! is_user_logged_in() || isset( $_COOKIE['bp-activity-scope'] ) ) {
        return ;
    }
    // additional check for activity dir
    if ( ! bp_is_activity_directory() ) {
        return ;
    }
    $tab = 'groups'; // 'all', 'friends', 'groups', 'mentions'
    // Set scope to our needed tab,
    // In this case, I am setting to the 'friends' tab
    setcookie( 'bp-activity-scope', $tab,null, '/' );
    $_COOKIE['bp-activity-scope'] = $tab;
}
add_action( 'bp_template_redirect', 'reign_set_default_activity_directory_tab' );