You can customize the workflow "CanLogin" to present a message box with your message in it. If you put the message box in the "CanLogin" workflow users will see it after they click the Login button and when they switch between projects using Tools > Change Project. You will have to add the customization to every individual project where you want the message to appear.
Refer to the Administrator Guide for information on workflow customization.
Refer to the Administrator Guide for information on workflow customization.