Thanks for the request. Currently there is a way to set this property through powershell for groups, but it only works for users in OWA. We have this in our plans for Outlook desktop as well and will be working on it.
Thanks.Regan Li commented
I am surprised to see there are only 28 comments about this issue here.
Is there another place we can raise our voice about this?
I just spend half of the day to search about this issue, and couldn't find anything from Google that setting up permission is not an option in Group calendars, and then I called the Microsoft support and found out about this article, https://answers.microsoft.com/en-us/msoffice/forum/all/office-365-group-calendars-permissions/d1aca62d-af1a-4eff-b500-afc0b0499059?auth=1
and it let me here. And there's no official KB about this!
how do you expect us to get away from the public calendar if Group calendar cannot even set up permission?