Multiple users edit a sighting?

How do I set permissions so that other people can edit one of my sightings?

I created some sightings of ash trees in a project for the Groton Invasive Species Committee. This data was meant to be the starting baseline. Now two years later, others have taken additional pictures, diameter measurements, and the like. How do they add the new data to the existing sightings? I don’t want them to have to give the data for me to enter it. That would be a pain for me, and create a bottleneck in any case.