be in on (something)
To be participating or involved in something, often something secret or exclusive. The conspiracy ran deep, and several members of the administration were in on it. Wow, that really was a surprise. Were you in on planning this?
in on, be
Also, get in on. Be or become a participant; be or become one of a group who have information. For example, Is she in on our secret? or I'd like to get in on this venture. [c. 1920] Also see barge in on; ground floor (get in on the); in good with.
be in on
be privy to a secret.