adding sub directories to a group?

[gnicholasuk]

I own a book exchange group, which I have just set up, and would be useful to be able to organise posts in genres of books. Ie, I would like to set up directories so that people can search for books by crime thriller, or biography etc etc. I dont mind organising the posts myself as they get submitted, it will just make it easier to use the site. I am completely new to all this, so can someone please advise!

Many thanks

Gareth