The phrase "in on" can be replaced by a variety of synonymous phrases, depending on the context in which it is used. For example, "in on the action" could be replaced with "participating" or "involved." "In on the joke" could be replaced with "aware" or "privy to." Similarly, "in on the plan" could be replaced with "consulted" or "included." Other possible synonyms for "in on" include "in the loop," "in the know," and "clued in." Using synonyms can add variety and nuance to your writing and speaking, helping to keep your audience engaged and interested.