Attendance I was one of the MCs of Aggie Pack during that game. Lots of excitement about moving to D1, lots of excitement with the new stadium, and a lot of work from two staff with a dedicated team of 12-15 sports marketing + promotions interns.
We dragged around 5-10 ancient IMac computers from the G-street annex to the stadium. We would staff the ticket office before the week before the game and on game day. Students swiped cards on a reader connected to these computers and we had a database that tracked each students attendance. First swipe was an Aggie Blue shirt. Then any other swipe was recorded, 3rd game was a beanie, 5th game sweat shirt etc. Targeted email blasts, Facebook groups, chalking, flyers, signs on stakes, we did it all for the bigger games.
This was almost 20 years ago. With the advent smart phones and abundance of tech savvy students it wouldn't take much to at least recreate some similar tracking.