adding a grid with specific class in adaptivetheme
AT does not use Grid classes embedded in markup (it does not use grid classes at all).

If you are asking this I assume you do not really understand how this theme works - you use the layout system (in theme settings, for the main container/page and panels layouts) and write content/element specific CSS yourself, aka use a semantic grid approach.

Grid classes in markup are a huge limitation - what seems so easy now, turns quickly into a massive roadblock later on.

answer by author of adaptivetheme http://drupal.org/node/1861948