I don't think there's any way to allow players to modify or delete text placed by anyone else. They can certainly add text, and edit any they placed themselves, but that's about it. A text field doesn't have permission fields to mess around with, so unless there's some clever API script for this I don't think it's achievable. They can mess about adding their own text until someone writes up a final version they're happy with, then the DM can move it to the map layer to make it "permanent"? Or you can promote all players to GM status?