Indeed, it seems there’s not yet a way to make BBPress read the BuddyPress groups and hide the froums belonging to private  groups I had to develop a way myself.

Furthermore, I needed to give forum moderation rights to the moderators of each group in BuddyPress on a per-forum basis.

This is a quick and dirty tutorial for future reference. Keep in mind I am using the bleeding edge (svn) versions of WordPress Mu, BuddyPress and BBPress.

At the end of this tutorial your BBpress will hide any forum associated with a non-public user group, and group moderators will have the needed  privileges in BBPress as well (without manual intervention).

Note: I assume you already work with those three softwares and you know how to install them, install plugins, etc.

First: install WordPress Mu 2.7 (it’s not stable yet, so you’ll have to get it somewhere from svn)

Second: Install BuddyPress bleeding edge version on top of WordPress.

Third: Download the svn version of BBPress and decompress it in a sub folder.

Be sure to properly integrate BBPress with WordPress. (further reading here)

When you get everything working, download the Hidden Forums plugin and install it in BBPress.

Now, open the hidden-forums/hidden-forums.php file and replace everything from the top to the “/*    stop  editing  here    */” mark with:

require_once( 'bp_integration.php' );

When you’re done, create a new file named “bp_integration.php” in the same folder and copy paste the following code. Save everything, activate, and you’re done .

<?php
//settings
global $bp;
$bp['groups']['table_name_groupmeta'] = 'wp_bp_groups_groupmeta';
$bp['groups']['table_name'] = 'wp_bp_groups';
$bp['groups']['table_name_members'] = 'wp_bp_groups_members';
$bp_integration_privileges = array(); //ids of forums the logged user can moderate
function bp_integration_apply_user_permissions()
{
    global $hidden_forums, $bb_current_user;
    global $bbdb, $bp;
    global $bp_integration_privileges;
    $query = "
    SELECT group.id as id, group.enable_forum as enable_forum, meta.meta_value as forum_id FROM `{$bp['groups']['table_name']}` as `group`
    INNER JOIN `{$bp['groups']['table_name_groupmeta']}` as meta
        ON group.id = meta.group_id AND meta.meta_key = 'forum_id' AND group.status != 'public'
    ";
    $private_groups = $bbdb->get_results( $query );
    $user_id=(!empty($bb_current_user)) ? intval($bb_current_user->ID) : 0;
    // hide these forums, list by comma seperated number
    $hidden_forums['hidden_forums']=array();
    foreach ( $private_groups as $group )
    {
        $hidden_forums['hidden_forums'][] = $group->forum_id;
    }
    $allowed_group = bp_integration_allowed_forums($user_id);
    $moderator_in = bp_integration_user_privileges($user_id);
    foreach ( $allowed_group as $allowed )
    {
        foreach ( $private_groups as $group )
        {
            if ( $group->id == $allowed->group_id && $group->enable_forum )
            {
                $hidden_forums['allow_users'][$group->forum_id][] = $user_id;
            }
        }
    }
    foreach ( $moderator_in as $moderated )
    {
        foreach ( $private_groups as $group )
        {
            if ( $group->id == $moderated->group_id && $group->enable_forum )
            {
                $bp_integration_privileges[] = $group->forum_id;
            }
        }
    }
    $hidden_forums['allow_roles']['all_forums']=array('keymaster');
    $hidden_forums['allow_users']['all_forums'] = array();
}
add_action('bb_init','bp_integration_apply_user_permissions',199);
function bp_integration_user_privileges ( $user_id, $limit = false, $page = false, $get_total = true, $as_col = false )
{
    global $bbdb, $bp;
    if ( $limit && $page )
        $pag_sql = $bbdb->prepare( " LIMIT %d, %d", intval( ( $page - 1 ) * $limit), intval( $limit ) );
			$group_sql = $bbdb->prepare( "
SELECT DISTINCT group_id
FROM " . $bp['groups']['table_name_members'] . "
WHERE user_id = %d AND inviter_id = 0 AND is_banned = 0
AND ( is_admin = 1 OR is_mod = 1 )
{$pag_sql}
", $user_id );
    return $paged_groups = $bbdb->get_results( $group_sql );
}
function bp_integration_allowed_forums( $user_id, $limit = false, $page = false, $get_total = true, $as_col = false ) {
		global $bbdb, $bp;
		if ( $limit && $page )
			$pag_sql = $bbdb->prepare( " LIMIT %d, %d", intval( ( $page - 1 ) * $limit), intval( $limit ) );
			$group_sql = $bbdb->prepare( "SELECT DISTINCT group_id FROM " . $bp['groups']['table_name_members'] . " WHERE user_id = %d AND inviter_id = 0 AND is_banned = 0{$pag_sql}", $user_id );
		return $paged_groups = $bbdb->get_results( $group_sql );
	}
function bp_integration_moderators_privileges ()
{
    global $bb_current_user, $forum,$topic_id;
    global $bp_integration_privileges;
    $args = func_get_args();
     $moderator_skills = array(
		'moderate' => true,
		'participate' => true,
		'manage_tags' => true,			//+
        /* ??!?! seems not work
		'delete_topics' => true,		//+
		'close_topics' => true,			//+
		'stick_topics' => true,			//+
		'move_topics' => false,			//+
         */
        /* ??!?! seems to work */
        'delete_topic' => true,		//+
		'close_topic' => true,			//+
		'stick_topic' => true,			//+
		'move_topic' => false,
		'view_by_ip' => false,			//+
		'edit_closed' => true,			//+
		'edit_deleted' => true,			//+
		'browse_deleted' => true,		//+
		'edit_others_tags' => true,		//+
		'edit_others_topics' => true,	//+
		'delete_posts' => true,			//+
		'throttle' => true,				//+
		'ignore_edit_lock' => true,		//+
		'edit_others_posts' => true,	//+
		'edit_favorites' => true,
		'edit_tags' => true,
		'edit_topics' => true,
		'edit_posts' => true,
		'edit_profile' => true,
		'write_topics' => true,
		'write_posts' => true,
		'change_password' => true,
		'read' => true
	);
    if ( ! in_array($args[1], array_keys($moderator_skills)) )
    {
        //echo '<pre>';var_dump( $args[1] );echo '</pre>';
        return $args[0];
    }
    if (!$bb_current_user || in_array('keymaster',$bb_current_user->roles) || in_array('administrator',$bb_current_user->roles))
    {
		return $args[0];
    }
    //--------> OK, LET'S GO !
    // prevents forum moderators to stick a topic to the forum's front page
    if ( $args[1] == 'stick_topic' )
    {
        global $super;
        $super = 0;
    }
	if (empty($forum)) {
		$topic = get_topic($topic_id);
		$forum_id = $topic->forum_id;
	} else {
        $forum_id = $forum->forum_id;
    }
    if ( !$forum_id ) return $args[0];
    if ( in_array( $forum_id, $bp_integration_privileges ))
    {
        $args[0] = $moderator_skills[ $args[1] ];
    }
    return $args[0];
}
add_filter('bb_current_user_can','bp_integration_moderators_privileges',10,25);
?>