A group I'm in gave up on someone I care about and forced him to quit. I feel like I failed him for not intervening earlier. This meant so much to him. How can I forgive myself?

The past cannot be changed - acknowledge that. You could have done something, but for some reason, you did not. Now that you feel bad about it, promise yourself that in the future, you will do what you can to stop anything unfair happening to your loved ones. We all make mistakes. That's what makes us human. Keep this as an experience and use this experience in your future affairs :)

Although you did not intervene earlier, it's not too late to do something about the situation. Being there for him shows that you care about this person and want to help despite not intervening earlier. Seeing that you can make a difference and ameliorate the situation are positive steps to take. However, I think you should ask yourself what you need in order to forgive yourself? Do you need your friend to say they forgive you or do you need to feel like you can change the situation? Only you can answer that, but you're not alone. We have all made mistakes in the past where we didn't speak up or try hard enough to prevent a situation. Learning from this and applying it to your life today is one crucial thing to take away from this experience. Forgiveness will come in the process :)