All of the code is compiled into dlls in the /bin folder, mojoPortal is a Web Application project type not a web site project type, we don't deploy source code in this model so just dropping in .cs files is not going to work.
Generally you should avoid forking the mojoPortal code. If you really need to make a custom member list page you should implement it in a custom project and then use a post build event to to copy yours over ours or else use a page with inline code instead of a code behind of you don't want to make a special project and then just replace ours with yours but the page must inherit from your own class or else from mojoBasePage and not from the class in the code behind for our member list page. In either case you will have to maintain your custom page and restore it after upgrades.
Alternatively you could avoid overwriting our file and make your file have a different url and then replace our memberlistlink control with your own custom one in the layout.master file of the skin. This approach would make upgrading easier because then our file won't overwrite your during upgrades requiring you to restore yours after upgrades.
My advice is to read the developer documentation before you start doing any custom development work so you can learn about recommended ways of doing things such as keeping all of your customizations in your own projects.
Hope that helps,