5
If you have a laptop, then, as you said, they can register locally and then you only have to export the newly created rows of the users table, and then import it in your real db.
Of course, if you site is still online in the mean time and new users register there, you might want to have a look at the uid field, as it might not be unique for the people who registered at your event. You'll only have to increment those uid by the number of people how registered to your online site.
Cheers !