Even if you could find something which would be an "index" of a method in the sense you understand it, it would be absolutely no use of it. Besides, the idea to find anything at all by method name is bad in principle.
To get some understanding on the related topics, you really need to learn Reflection:
You can actually find out everything about each method in each compiled assembly. However, finding any metadata based on string information such as the name of method, type, property, etc., would make really bad code, in terms of its maintenance, well, in most cases.
A really good approach is to implement some interface, share the interface definition with host assembly (you can defined it in a host assembly and reference this assembly by dynamically loaded assemblies, because there is nothing wrong in referencing the entry assembly, or application assembly (EXE); I'm not sure you have enough knowledge to understand this part, but you can always ask your follow-up questions) and find the type by the interface it implements. If this is done, you can instantiate the type using Reflection and then directly use interface methods, all of them.
I don't provide further detail just because I don't want to waste time — I have no idea of your intended purpose. That said, you should start all your questions with your ultimate goals. Chances are, you won't need Reflection or other advanced stuff at all, for the time being…