Dynamics GP 2013 R2 includes the ability to notify users. Typically this is for down time, but it could be for anything.
In R2, select Administration>Utilities>Send Users Message
The display shows all users and any companies that they are logged into. You can choose to send logged in users a message or add a message as a task with a reminder.
This is a great way to remind users that GP is going down for maintenance. Conveniently, locking users out for maintenance is located right below it on the menu.