Unless they are doing things differently this year, it is indeed a 4 person scramble format with everyone on the same card. Just sign your team up at the start and I believe they ask for a team captain. I can't remember exactly how they assign cards, in the past I always requested to be on the same hole as another group that I knew and it wasn't a problem.
I've done a couple of these benefit tourneys out at Thornfield and they are a blast. It's a laid back, honor system type of tournament played on a fantastic course with great amenities. If you haven't got a chance to play there I highly recommend it.
As an FYI - if you can't make this one the Ozanam one is on the 8th of September.