When posting an event announcement, the event node author can add co-organizers of the event using User reference fields. It would be unbelievably helpful if the listed event co-organizers could also edit and update the event details.
The Node access user reference module may be just the ticket. It controls node access grants for user accounts listed in User reference fields:
It's well-maintained and as of today is used on 2148 sites. I'm available to do a fair amount of testing to help make this happen.