There's nothing out of the box that I know of - this might work though.
Generate a list of Personalized URLs for the people registered:
Then create a QR code for each PURL, which you'll put on people's badges:
Then you could create a smart campaign with the trigger "Visits Web Page", and the filter Anonymous IP (set to the IP address(es) of the device you're using to scan the QR codes, I assume an employee's smart phone), so people can't just scan it themselves to check in. In the flow, change the program status to Attended (or whatever status you want to change them to when you scan them).
Be sure to clear the cookies on the device afterward, otherwise your web activity could be tracked as one of the leads.
Thanks, Grant. We're specifically looking for a way to scan and import new leads who aren't on our attendee list. The Marketo app is great for just checking off people who are already registered, but we don't have a good way of getting completely new people into the system. Any ideas?