The phrase "were in on" can be replaced by a variety of synonyms depending on the context of the sentence. Some synonyms for this phrase include "participated in," "were involved in," "had knowledge of," "were privy to," "were in the loop," "contributed to," "had a hand in," "were aware of," and "were in collusion with." These synonyms convey a sense of being included or involved in something, whether it's a project, plan, or scheme. Depending on the tone of the sentence, these synonyms can also imply a level of guilt or complicity in a questionable or illegal activity.