bbPress.org

users can subscribe to forums and topics, both work the same way. The post ID is either a forum or topic.

Not suer what you mean by command – if sql then I cannot help you. If php then you would use get_users and loop through that and use

bbp_add_user_subscription( $user_id, $forum_id )

private groups – the code simply checks matches. Users have a user_meta entry of ‘private_group’ and forums have a post_meta of ‘_private_group’ the general settings are stored in the options table