I'd say in the text "Written by the University of Georgia Vigre group" and then list all the people in the bibliography or just say "by the 28 authors listed at the arXiv."
For the AIM workshop again I'd consider "by the AIM workshop on subject X." I'd also consider X,Y, et al. where X and Y were the organizers of the workshop. I think it's ok to use et al if there's some non-alphabetical way of assigning more credit to some people.