"Groupthinked" is a non-existent word in the English language. However, if we break it down into two separate words, "groupthink" and "ed," we can find its antonyms. Groupthink refers to a situation where a group of people adopt the same opinion or belief without any critical evaluation. The antonyms of groupthink can be independent thinking, critical thinking, or individualism. On the other hand, "ed" is a suffix that means "past tense," so the antonym would be the present or future tense of the verb. To rephrase the sentence without the made-up word, we could say, "The group engaged in groupthink" or "The group was prone to groupthink.