I shared a map on Maps Engine with anybody to edit. However, when I open the link without signing in to my Google account, I am not able to edit the map. Is it possible to collaborate without requiring signin?
I found that I was having issues sharing maps too for a while. I found that the solution to this was to share the map from the page where you were editing it (and not from your drive), as for some reason you get slightly different URLs.
You should also select the option for people to be able to "Can edit" from the drop down menu.