let in on

/L EH1 T IH0 N AA0 N/
verb
  1. 1

    (ditransitive) To disclose information to someone; to tell somebody a secret or share privileged information.

    โ€œDo you think we should let him in on the shortcut?โ€

Source: https://en.wiktionary.org/wiki/let%20in%20on