I am looking to add a custom button to the Account list view that opens a Lightning component. How can I do this?
You can not call a Lightning Component from List Button directly. You need to do it using visual force page. You can check the Component developer guide for this. Complete solution is given there.
Another approach could be overriding a standard action (like view, new, edit) with your Lightning Component as described here:
Not sure if that fits your use case as overriding will be global and not only triggered from a list view.