Since you're talking about ASP.NET you can have the SignalR service by ASP.NET. Which is dedicated for such realtime updates accross the network.
All the users across the system would be able to chat and you can control the messages by showing them to only a number of users.
This way you can use the pure ASP.NET help and create a system for yourself.
If you don't wanna use this, then you have many other options. You can either use a Third-Party software. Or you can create one of your own.
To create one of your own, you'd require some skills in programming. Such as basic ones, as how to program in ASP.NET, and how to use a Database. In the Database you would create the tables and there you would save the values the users would pass on to you. After that, you'll simply just show them on the Web page. It would just require a simple Database.