Cna't find anything definitive on, was this something no one wanted to talk about or, was he a bit player on the sideline who had little involvement/little impact on the witch hunt that occurred?
When called before HUAC in 1947, Reagan refused to give names, saying that he had "no investigative power" as head of the Screen Actors Guild to find out who, if any, among its members were actual members of the Communist Party. He did say there was a "clique" that members often associated with Communism, but that he believed the Union had them under control. You can read his testimony in full [here.] (http://historymatters.gmu.edu/d/6458/)