In order to look back on what actions were taken over time, most major system events are logged for you. For example if a user has been created or deleted, a broadcast has been sent, or someone has been assigned to a group.
To view system activity navigate to the activity section from the main navigation (Admin > Activity). You can download the past 30 days activity from here. Clicking on the activity number will bring up a window of additional information if available for that particular event.