How do I see an audit trail of when users were granted NetSuite roles

For an individual user, open the employee record, go to the Access tab and view the History subtab. This subtab will provide a history of all global permission or role changes made to that employee. For multiple users, run a search against the 'Employee' record type and add the following fields to view role changes: Role Change (the affected role); Role Change Action (whether the role was added or deleted from the given user); and Role Change Date (when the change was made).

Fastpath has a report in its 'audit trail' product that logs and reports all changes to user access. The report can be run for a given time frame. My habit is to run and archive the reports each quarter.