Robin W

This Plugin changes the ‘freshness ‘ that bbPress displays on topic and forum lists (eg 4 hours ago) to the date of the last post to that forum or topic (eg 17th January 2014 at 5.15pm).
It also allows you to set the heading for this new date format.
The existing (bbpress) Recent Topics widget works with date, but a revised Recent Replies Widget to display the date format called (Last Post) Recent Replies is included.

bbp last post

After trying to code things myself (to keep the number of plugins down), I found it easier simply to install this.

Very happy, it works perfectly

For bbPress - changes the freshness displays to date

This Plugin IS NO LONGER MAINTAINED
use https://wordpress.org/plugins/bbp-private-groups/
This plugin displays private forums titles to non logged-in users in main index.
This allows non logged-in users to see that forums exist, but have no acces to these.
If selected by a non logged-in user, the use is sent to a page (determined in settings), allowing the admin to determine what information is given, eg login, registration, text etc.
It also has the options to
display forum content (descriptions) for sub forums
remove the prefix ‘private’ from all private forums
Works with bbpress 2.5.4

Private forums visibility

For bbPress - displays private forums titles and optional descriptions to non-logged in users, and optionally hides the prefix 'private'

bbp last post vs Private forums visibility