I'm sorry for the situation they went through, but this is normal for conventions/gatherings/etc. They only have 'registration' during certain times. If you don't make it during those times, you don't get in. Tickets or not. Been through this a few times myself. He was basically breaking the law when he tried to push the issue and step through the door. It was the guards responsibility to act because it would have been on them if they'd let him in and then games were destroyed and/or stolen. It also isn't fair to assume someone is going to man ticketing 24 hours (although it would be nice if that was possible).
The lesson learned here though is that whoever handles the groupon (or any ticketing) for it, needs to have them put on the registration hours.