Hey Dan,
This article provides an overview of how to use dynamic content in Marketo: https://community.marketo.com/MarketoTutorial?id=kA250000000Kz1pCAC You would likely need to create a segmentation with an "Email address is not empty" for the top segment, and then nothing for the default. Since anonymous leads are always shown default content, this would allow to to creae one piece for known leads and one for the anonymous.
As far as leads being in the same location, this has little impact, as Marketo's dynamic content is served based on a cookie in the browser, and not based on ip address.