Currently users have the ability to create new entries in the Office 365 group calendar in both Outlook desktop client and Outlook on the Web (OWA).

You are able to copy that same entry from the Office 365 group calendar your personal Calendar via 'Drag & Drop' or 'Ctrl+C'-->'Ctrl+V' on that entry via the Outlook desktop client.

You are unable to copy and paste a calendar event from your personal calendar to the Office 365 Group Calendar via any of the mentioned methods. You will be met with the following message:

"Cannot copy the items. you don't have appropriate permission to perform this operation."

Your only choice is to re-create that event on the group calendar.

"With an Office 365 group, you don’t have to manually assign permissions to each of these resources, because adding members to the group automatically gives them the permissions they need to the tools that the group provides."
--https://docs.microsoft.com/en-us/office365/admin/create-groups/office-365-groups?view=o365-worldwide

and yet, this doesn't seem to be the case based on that message...

Suggestion - allow for Calendar events to be able to be 'copied' from personal Calendars to Group calendars.

Example case:
"We have a team in Chicago who was managing their shared O365 Group calendar by having associate drag and drop their time off into the group calendar.Doing it that way allowed the invite to be added to the group calendar without sending everyone in the group an Outlook invite."

This was a feature that apparently worked that no longer works. This should be a ticket with MS support but I figured if it isn't advertised in UserVoice, and it wasn't an advertised feature, it should be.