Admin can send a message which will be displayed to all website users.

1. Install / copy Admin message module.
2. Login with admin's user.
3. Go to Administer -> Site building -> Blocks.
4. Assign region to Admin messages block. (This is where the message will be displayed).
5. Go to Create Content and click Admin message link.
6. Write down message Title and Message.
5. Select submit button.
6. Login out and sign in with any other user. The admin's message will be shown in specified region.