You make sure that everybody feels safe speaking out and different opinions are wanted to be heard and thoughtfully discussed are obvious first steps. It’s not easy, but anonymous regular surveys help from own experience catching unhealthy environments early. Reddit is the cesspool of groupthink because of wrong reward systems and poor moderation. Everybody can (and must) do their part here. If you feel someone might not have been heard out or dismissed (correctly or not), ask follow-up questions instead of just moving on.