Hi Rob, on the Users page under System Administration you can export the list as a CSV in the upper right:
You can then open that in Excel or your spreadsheet of choice, and copy/paste the emails into an email program. Other options to consider would be to make a custom app on a shared dashboard to give tips, or there's the Disclaimer Message to put a callout at the top of Agile Central Configuring the Disclaimer Message in Agile Central
Let me know if you have questions/comments with any of these.
-William