The diplomacy side of the game is very limited at this time. You cannot initiate any sort of diplomatic action as you might expect with any other 4X game, instead you have the DR (diplomatic rating) system wherby every action you take either increases or decreases your DR, a higher score means better relations and allows you to use certain things such as allowing an NPR to trade with you etc. In addition you have two sets of DR, one is how you view the NPR the other is how they view you, so while your general society may like an NPR and have something akin to open boarders, they may see you all as squishy meat bags not to be trusted.

The following pages give a more in depth look at how it works and how you can take certain actions to manipulate the DR if you have a certain goal in mind.

http://aurorawiki.pentarch.org/index.php?title=Diplomacy
http://aurorawiki.pentarch.org/index.php?title=Diplomacy_Rules