This might have something to do with it. Is it a school production, or an after school activity or some combination? I think it not being a straight school event explains a lot. A non-teacher outsider would probably think a shy kid wouldn't want to do much on the show.
Regardless, I think following up is a good idea (and I still think you should stress how much your DD wants to be involved and even have her just show up and say "how can I help" might be a good idea although I get she's shy...at some point she has to get past that, it's not serving her). I really hope it works out for your DD.