Is it possible to work together in society and still live individual lives? Of course! Steinbeck did not devalue the idea of individualism; he valued and respected the individual. What he rejected was an economic system in which those with money and power exploited and abused those without means and influence. This was the same philosophy that once drove emigrants from the Old World into the American colonies. (Read Crevecoeur's "Letters from an American Farmer.")
When The Grapes of Wrath was published, it laid bare the abuse of economic power as Steinbeck observed it, personally. Naturally, those who wielded that economic power (especially in Oklahoma and California where the book was banned) were outraged because their practices had been challenged. Often, when working men and women seek to form a union to even the odds and better their working conditions, those in power cry "Socialism!" In Steinbeck's time, they cried "Communism!" Discussions of socialism and communism, of course, serve to divert attention from the real issues.
Steinbeck believed in the individual and the individual American Dream. Look at George and Lennie in Of Mice and Men. They did not dream of working in a commune, side by side with others; they wanted a piece of land that belonged to them where they could live their individual lives in peace and security.