Movie grouping

Hi!

I love how XBMC groups my James Bond or Alien movies in a group, but I'm wondering how can I create my own grouping or edit one that exist already.
For example, I have the 12 Gamera dvd box set, but XBMC doesn't identify those as a group. I also have nearly all the Godzilla movies but only 4 show up in the Godzilla grouping. Is there a way to create or edit those group?

Thanks.