Let in on
Definition 1 of 1
Pro Tip 1/3Sharing Secrets
Used when someone is included in confidential knowledge that is not yet public. 
Finally, my siblings let me in on their surprise party plans for our parents.
Pro Tip 2/3Trust Implication
Implies a level of trust towards the person being informed. 
My mentor let me in on his methods only after I earned his trust.
Pro Tip 3/3Exclusive Information
Suggests that the information is exclusive or limited to certain people. 
The director let me in on the upcoming changes before they were announced.