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?
be in on
be privy to a secret.be/get ˈin on something
(informal) knowing about something; included in something: I’d like to be in on this project if you’ll have me.She wants to get in on what the others are doing.Shall we let him in on the secret?