Candy is isolated because he is an old man that is missing a hand. He can't do a lot of work because of his missing hand. There is a scene in Of Mice and Men where Carlson bullies Candy because of Candy's dog. Candy feels intimidated and let's Carlson kill his dog. Candy feels very sad and he doesn't talk for almost the rest of the night. This scene shows that Candy can easily be intimidated. Here is a link for that scene in the Of Mice and Men movie.
http://movieclips.com/watch/of-mice-and-men-1992/candys-old-dog/
Curley's wife is isolated because she can't be trusted. Everyone on the ranch thinks she is a slut. Everyone on the ranch tries to avoid Curley's wife because they know that Curley's wife might get them into trouble. All Curley's wife would have to do to get them in trouble would be to say that one of them touched her. If she did that, Curley could get them fired.
Lennie is isolated because he is not smart at all. He is about as smart as a child. Lennie can't even speak properly. George completely guides Lennie through life. Without George, Lennie would have nothing. Not a lot of people at the ranch talk to Lennie. Lennie is not smart enough to have a good conversation with anyone.