The story focuses on the character of Tom Joad as an individual and the Joad family as a collective unit. Both are important to the story.
It is Tom who we see initially, as he makes it back home after getting out of jail. It is through Tom that the story expresses its theme of individual struggle. Most of the other characters are important as part of the group that is making its way from Oklahoma to California.
Tom is always a little bit separated from the group. His attitude is a little angrier, a little more sensitive to the unfairness of their current predicament, and a little less willing to accept what has happened to them. It is Tom who goes off and finds out that Casey and the others are trying to change things by organizing, and it is Tom who kills one of the strikebreakers. This puts him in a unique situation. When the group leaves the government camp at the end of the story, Tom has to go his own way to avoid capture. He will have to make his way however he can, without the help of the others, who can lean on each other for help and guidance.
Steinbeck makes it clear that it is through group effort, sticking together to strike and bargain, that the workers will achieve a better life. However, people like Tom will still have to struggle to find their own way in the world.