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.
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It's now 2020 - please make read-only an option in Group calendars for admins. Also, it would be great to be able to choose select members of a Group to attend a meeting and not the whole Group. We want to show the event/meeting on the calendar but not all members need to attend each meeting.