How can I add a user to just one sitemap for editing?

Invite your user here: https://app.dynomapper.com/user-access
On the same page, click the key next to the user and change the preferences of that user to:

Sitemap Access: Off
Create Sitemap: Off
Delete Sitemap: Off

This will default the user not to see all of your sitemaps. The user will not be able to create new sitemaps or also delete your sitemap. 

Then, in the sitemap menu: https://app.dynomapper.com/sitemaps
Find the sitemap that you would like to share the user, Click Sitemap Permissions and share that one sitemap with the user. Please let us know if you run into any issues.