Audit group membership or mailbox

[iamdp]

Running Exch 2016 on prem,
Someone add the user mailbox to a group which was not supposed to be added. How do i find the audit history of the mailbox or the Group to know when and who added particular user.

Thanks 

[#2]

Group changes are audited in the Admin audit log: https://docs.microsoft.com/en-us/exchange/security-and-compliance/exchange-auditing-reports/view-adm...