You could show a one-time notification window when people log in with that kind of information, they can click it away or maybe even have to check 'don't show this again next time'.
This does depend on how active your user base is. If they are already logged on they will not receive the takedown notice when you use just the log-in notice. This can be circumvented by planning ahead atleast 1 day, but you still would like to inform your userbase like 15 or 30 mins in advance. However to achieve that you are stuck with implementing some sort of chat functionality.
All pro's and cons taken i would go with the login suggesting Bas made over any chat service.
A very simple approach is to create a simple message object and show this to all users after login. You need to set a boolean in your user object which you will use to determine if the user has seen the new message.
When you set this boolean by default on false, you can check this value in your navigation microflow for each user. When you add a new system message, you need to retrieve all users and set the message boolean on true. In the navigation microflow check this boolean. If it's false, skip the message and do whatever you want, if it's true show the message in a form and then redirect the user. When the message is shown to the user, set the boolean on false. This way the user will only see the message once.