as far as I can work out, there are no issues with private groups as such, but upgrading to 4.4 does produce a bbpress issue.
At the moment I think the following is true
If you are staying with 4.4 then implement the following
open wp-includes/post.php, scroll to about line 4300 to find function get_page_uri( $page )
In that function replace
if ( ‘publish’ === $parent->post_status ) {
with
if ( ! in_array( $parent->post_status, get_post_stati( array( ‘internal’ => true ) ) ) ){
If you revert to 4.3.x, then the upgrade will have altered the database, so you need to go into each forum and toggle visibility public/private status , saving in between. So if private, toggle to public and save and then toggle back and save again. If public, do the reverse.
Not sure of you need to do that if you stay with 4.4
Should all be fixed in 4.4.1.
If anyone thinks different, please come back – would be good to have a single definitive post saying what users should do