I can't speak for all the groups, but I remember recording while on tour. I believe we did our session during Preview of Champions weekend in Ypsilanti.
For Spirit that year, "Why do the Nations..." was our show opener, so the recording was the full corps.(Our arrangement, obviously.) I thnk ensemble size varied on the CD, with some corps only using small ensembles.
Again, I can't speak for everyone, but there were no "ringers" involved.
Hope this helps...I always liked that CD!