FEP-c7d3 is a work in progress, and it will probably move away from relying on attributedTo for authentication. But there might be other reasons to put a single actor there. For example, which actor should be in actor field of Offer activity? I think it should match Event.attributedTo.
I think signed Accept(Offer) activity can be used as a proof. You can attach a collection of those to your Event as Event.organizersAccepted, and everyone who views the Event will be able to verify that specified organizers are consenting.
Alternatively, Event.organizersAccepted may contain only activity IDs to be fetched. In this case Accept activities donât need to be signed.