I always felt (as a kid, and today) like Han, Luke, Leia, etc. were all friends as well. I actually just watched ESB again recently, and they use the word "friend" to describe that relationship a fair amount. When Luke is missing on Hoth and one of the Troops says to Han that "the temperature is dropping to rapidly", he responds "that's right and my friend is out in it". Then of course there is Luke's visions during his training with Yoda: "I saw a city in the clouds"...."friends you have there". I always felt like, particularly starting when they return from the Death Star in ANH and Han asks Luke to come with him that they were considered friends, and it built from there. Luke seemed disappointed that Han wasn't staying, and Han offered Luke the opportunity to come with him and Chewie. Like others have mentioned, I think others like Lando, Wedge, and of course Chewie and Leia are all included in that circle of friends.