"Atonement" applies to most every character in the novel: they're all trying to achieve it in some way or another. The way it applies to some is simply more obvious than others.

In the case of Robbie, he's obviously trying to achieve atonement for something he didn't do: he wants to be proven innocent, so that people don't think he's the rapist anymore. He was never able to achieve this, though, because Briony never told them what really happened. Lola's atonement was much more subtle, though. I believe she knew it was Paul the whole time, and that's actually the reason that she married him: she wants to atone for the fact that she was raped. Basically, she married him because she felt she became 'worthless' after the rape. Of course, this doesn't seem like it would work; it actually seems counterproductive. It's very likely that Paul would only continue to abuse her after the marriage.

Briony's experience was the most important, though; hers was the basis for the entire book. Briony spends her entire life trying to atone for what she did to Robbie. It's the reason she wrote the book: to show what she wishes she would have done to fix the problem. Of course, this doesn't fix the problem, at best it might make her feel better.

I don't feel sorry for her. Yes, she must have been miserable because of what she did, but she should. It is her fault; and as soon as she realized what she had done, she should have done whatever she could to fix it.