Check System.Addin Namespace on MSDN
Start with this. Then I also recommend that you look at Mono.Addins . You can even look at the Mono.Addins SVN chest to see how implementations work internally. You will need to understand how interfaces and attributes play a vital role in creating your plugin architecture.
The Managed Extensibility Framework is still under development, but you should become familiar with it, as Johannes points out. If your team is not comfortable moving forward with MEF for your production project, at least have some knowledge within.