click on one of their outposts, and then click on the little 'i' button in the top right hand corner, and then press the heart button again in the top right hand corner to become friends with them. I don't know if you can see your friends list or whether you can contact them outside of a game. Also I don't you can search for players outside of games so I don't think friending by that method is valid either, unfortunately....
Kings aren't OP
"Imagination is more important than knowledge"
----Albert Einstein