There is a 3rd party iPhone app for taking meetup attendance. You tap the face of people who are attending and it checkmarks the person.
But as mentioned, you still cannot mark folks as "no-show" via the app or api. But it does allow you to take attendance quickly and then reconcile your no-shows at your desktop later.
One issue is it doesn't show you the attendance info for past meetups that are a certain age, so you have to rush home to update your noshows. I've sent an email to the developer and hoping that feature will be added.
Joe