If someone doesn't have a name tag/business card, have a notepad, ask them to write their name, and hold it up with everyone else's cards (in order l-r)
We tried the 'take names on a legal pad and try to match to the shot number' method, but was a lot of work and if one was wrong it made several wrong and had to go back and redo it. The next time, I used small paper tablets (the sammer white legal pads) and a Sharpie, explained to them how we wanted accuracy, and asked them to write on the pad exactly how they wanted their name listed with the group photo - some didn't want their last name, only the last initial, some had prefixes or suffixs (ie: Dr., Jr., D.C. etc.) we took a photo with the pads, and a couple without. Once we explained how we are dealing with a large number of people in a short time and our desire at having their photo and name be accurate, they had no problem with the pad idea.
It made it so much easier to keep track of who was who, their names were right, and an added benefit, we could identify most of the people in the candid shots as well. I thought about using small dry-write boards next time, but keep thinking there might be a reflectivity problem from the board surface. Will have to try it sometime I guess.