Open the message.
On the left upper corner you should see a image with a "?" in it, meaning that this email is not in your contacts. Click on the image and you get a popup which gives you the choice to add the email to a new or existing contact.
I am not sure if this function is yet available in the latest Google:play version, i am running the latest developer build 1.2.4.27 which you can download here in the forum.