The Staff roles and groups history will provide you with a history on permission and assignment changes for Roles and Staff Groups. Any Staff user that makes changes to Roles or Groups will be logged on this page. 

Note: Only System Managers can make direct changes to a Staff Role. However, if a Staff user has the Setup privilege, they have the ability to grant Process permissions to both Roles and Staff Groups. 

Top Menu >> Users >> select the Reports tab >> Staff roles and groups history

You can export this report by clicking the download button on the top right of the page.