Instead of an email with the Check-in link, allow users to put in a mobile phone#. Send the 'It's Time To Check In' message via SMS with a link the user can use to check in that way. This way users won't have to go all the way into email their mobile devices (which requires multiple authentication steps) if they're running late and need to check in (something that actually happened to my boss this morning when he was delayed at a doctor's visit).