I added a new folder to mirror an NNTP group, but I am not seeing any posts yet.

This assumes you are properly configured (NNTP Control panel, and/or folder level NNTP options as required) and authorized to access a news feed.

Depending on the source of the feed: push versus pull, the type of pull options the NNTP feed server provides, bandwidth, and the number of posts that must be brought in to web crossing and processed, there can be a significant delay (several minutes or more) between the time you create the group and when you begin to see posts appear.