An authenticated user can see the Last Post column with the date and time, but there isn't a user name included with that info. If I am logged in as the admin, I do see it. My users would like to see that from the list view versus going into each discussion.
I don't know if this is the best approach, but when I set it up, I changed the permissions, allowing the authenticated user to add discussions, reply, and some others. I don't see any permissions that relate to the user who last posted.
Is this a permission, a setting, or a problem?