The core functionality should just be a simple database that collects inputs from a form. Isn’t that what an RSVP is? Yet in order to use, I have to input names and emails (which may be a useful feature for some). The plugin then does some matching based on names which can run into trouble when you have people with a formal name and a nick-name. You see where I am going with this? It is really just too much code for the base functionality. I understand that you have more advanced features, but keeping those separate from the core functionality is just better architecture (you can then test them individually). I imagine that you find it difficult to add new features and debug since changing one part of the code ends up breaking other bits.
Have a look at the Akismet Anti-Spam plugin; it may provide you with some ideas for refactoring.
I hope you find the feedback helpful, because my intent is to help move you towards a better quality product.
