https://www.airbnb.com/help/article/2701/extenuating-circumstances-policy-and-the-coronavirus-covid1...
this policy says that they can be refunded if the "guest" or "host" has covid, doesn't specifically indicate if ADDITIONAL guests or staff or family members or other folks affiliated with your listing are included or not.
If it were me, I'd tell them that I'm sorry their group has run into an issue, but that the cancellation policy will apply. They do receive the cleaning fee back if they cancel and they could perhaps get abb to refund any taxes paid. And if you wanted them to cancel sooner, rather than later (if they cancel prior to 24 hours before reservation begins then they don't get to review you!!) then it can be worthwhile to offer them a time sensitive bonus (I'll refund an additional 20% above the cleaning fee if you cancel before 5pm today, etc)