If I wanted to separate code in the Members section from the main site can I just create a new project and drop the new dll into the bin folder and the files where they go when I'm done?

Can you elaborate? Is your goal to break out a part of one web application? You are going to run into problems if you try that. Explain what you want to do in more detail.

Right now half of the size of the dll is from the members section and I was wanting to reduce it and make it where I can work on one part of the site that won't affect the other. ie. uploading the DLL file will cause an error if someone requests a page. If there are 2 dlls I'll have half a chance of affecting a user's experience.

Well, given that all the code in a project gets compiled into one assembly, you can't easily break apart one web project. I suppose you could create a separate project for the members section. You just have to make sure you don't get into a situation where each project requires access to classes in the other. Then you'll get yourself into a circular reference problem. Of course, building 1 site out of two different web application projects could get you into other kinds of trouble.

The reason I asked is because I changed the template of an existing site but kept the old site there. The old stuff still works and I was wondering if I could keep it seperated like that. How would I call a class from a different assembly?

If you want to use a class from another assembly, that assembly needs to be referenced by the project. Then you can use the classes as you always have, but you'll need to use the full name of the class (including namespace) or use an Imports/using statement at the top of the code file where you will consume the other class.

Let's say you have a project called "MyClassLibrary" with a class in it called "MyClass". You need to do either this:

Dim objMyClass As MyClassLibrary.MyClass

   OR

Using MyClassLibrary
...
Dim objMyClass As MyClass