be/get in on something

get in on

1. To become involved with, or participate in, some group, organization, or activity. Let me know if you'd like to get in on our party this Saturday. Why don't you go get in on the fun with the other kids?
2. To be or become privy to special or secret information, among a select group. I'm looking to get in on the hot trading tips in investment banking.
See also: get, on

be in on

be privy to a secret.
See also: on

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?
See also: get, on, something