Hi,

this will require you to do some coding around the discussion repository, you will have to override the discussion topic web part data binding method, then you will have to inherit the discussion repository and override few methods to change filters. Next step would be to create a filter that will fetch the group topics only from public groups, only from discussion boards that current user have permissions to and include the main filter then append new topics sorter.

As there are many complex filters involved in this process this is exact scenario where is much better to have a source code package to better understand how to do everything or even change the code directly.

Regards