Regarding an older question: 

How do I give some people limited time access to Mapshare

https://forums.garmin.com/outdoor-recreation/inreach/f/inreach-general/300334/how-do-i-give-some-people-limited-time-access-to-mapshare#pifragment-1296=2

I'm not sure when this feature was impleented, but you can now change the URL of your MapShare link.  

See https://support.garmin.com/en-US/?faq=0ZJe6RK2FZ7OYUq3HkXVx9

So although you can't give different levels of access to different users, you could in theory create a new mapShare URL for a single trip (say, a trip you want the public to view) and then change the URL when you were finished and wanted to go back to a more private group of users.