The question of “can I give edit access to one of my players?” comes up reasonably often. I have a plan for allowing this (that appeared in one of my long-term roadmaps even).
I will add access controls such that you can give edit access to particular players. I would also like to have more options around gating access to a game besides just “if you get the URL you have access to it forever”.
In the meantime, if you’d like to share edit access to a map with another user (like a co-DM). You can currently do this by creating an account together, and creating the map with that account. Both DMs will be able to edit the Shmeppy map without problem (Shmeppy is smart enough to know how to blend together changes from two separate computers even if you manage to make two changes simultaneously).