Using "orderby" in TPG line

Hi!

I am using this plugin on a site I’m working on and am curious what options are available as part of the “orderby” option? Right now I have items being ordered by titles (which consist of names of individuals). But it’s ordering by firstname instead of lastnames, so I’m trying to find a workaround to make them sort by last name only. 🙂

Thanks in advance for your assistance!
-Lynn

http://wordpress.org/extend/plugins/tpg-get-posts/