Hi Lauren,
That would depend on what you mean by "responded to emails".
As you can imagine all that Marketo can really track is who is opening emails, clicking links in emails and if everything is set up correctly, what the people who click on your email then do online/website. I would suggest you think about what this data is for so that you can work out what report to do. My experience is that opening an email is a terrible report as many emails are opened "automatically" and therefore that report does not provide much value. The best report would be if you can plan your emails so that there is a specific web page you want visited so that you can track who does that (like submitting a form, viewing pricing etc.)
You can very easily pull lists of who is opening and clicking on your emails by pulling of smart lists. If you are asking users to click on links in your emails and have the Marketo tracking code on the pages they are visiting you can also then report on that activity. For instance you could track who visited maybe a pricing page after being sent an email.
All this would need to be done through smart lists using activity criteria like visited web page, clicked link in email, openened email.
Simon