Search the web for plugin architecture .net or the like and you'll find various examples. There's likely some on this very site. Basically, you create a DLL that contains an interface and all plugins must implement, often named IPlugin. You will then need a reference to that DLL in your application project and you develop to that. Each plugin author will also need a reference to that DLL in there own project and they then implement your interface. At run time, you find the DLL and use Reflection to interrogate it to see if it contains a type that implements IPlugin. If it does you load it and interact with it via the IPlugin interface.

The .NET Framework also has an add-in framework built in, in the System.AddIn namespace, but I have no experience with that.